Surface Modification of Textiles

Surface modification of textiles

The Textile Institute and Woodhead Publishing The Textile Institute is a unique organisation in textiles, clothing and footwear. Incorporated in England by a Royal Charter granted in 1925, the Institute has individual and corporate members in over 90 countries. The aim of the Institute is to facilitate learning, recognise achievement, reward excellence and disseminate information within the global textiles, clothing and footwear industries. Historically, The Textile Institute has published books of interest to its members and the textile industry. To maintain this policy, the Institute has entered into partnership with Woodhead Publishing Limited to ensure that Institute members and the textile industry continue to have access to high calibre titles on textile science and technology. Most Woodhead titles on textiles are now published in collaboration with The Textile Institute. Through this arrangement, the Institute provides an Editorial Board which advises Woodhead on appropriate titles for future publication and suggests possible editors and authors for these books. Each book published under this arrangement carries the Institute’s logo. Woodhead books published in collaboration with The Textile Institute are offered to Textile Institute members at a substantial discount. These books, together with those published by The Textile Institute that are still in print, are offered on the Woodhead web site at: www.woodheadpublishing.com. Textile Institute books still in print are also available directly from the Institute’s web site at: www.textileinstitutebooks.com. A list of Woodhead books on textile science and technology, most of which have been published in collaboration withThe Textile Institute, can be found towards the end of the contents pages.

Woodhead Publishing in Textiles: Number 97

Surface modification of textiles Edited by Q. Wei

CRC Press Boca Raton Boston New York Washington, DC

Oxford

Cambridge

New Delhi

Published by Woodhead Publishing Limited in association with The Textile Institute Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington Cambridge CB21 6AH, UK Published by Woodhead Publishing Limited in association with The Textile Institute www.woodheadpublishing.com Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington Woodhead Publishing Cambridge CB21 6AH,India UK Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishing.com www.woodheadpublishingindia.com Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Published inNew North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Daryaganj, Delhi – 110002, India Suite 300, Boca Raton, FL 33487, USA www.woodheadpublishingindia.com First published 2009, Woodhead Publishing Limited and CRC Press Published in North America by CRC Press LLC, 6000 Broken SoundLLC Parkway, NW, © Woodhead Publishing Limited, Suite 300, Boca Raton, FL 33487, 2009 USA The authors have asserted their moral rights. 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Contents

Contributor contact details Woodhead Publishing in Textiles Preface Preface 1

Surface modification and preparation techniques for textile materials

xi xiv xix xix

1

M. J. JOHN AND R. D. ANANDJIWALA, CSIR, South Africa

11.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2

Introduction Natural fibres Synthetic fibres Surface preparation techniques for textile materials Surface modification techniques for textile materials Recent studies on the modification of textiles Future trends References

1 2 4 5 7 9 22 23

Textile surface characterization methods

26

Q. WEI, F. HUANG AND Y. CAI, Jiangnan University, China

2.1 2.2 2.3 2.4 2.5 2.6

Introduction Surface characterization by advanced microscopies Surface characterization by advanced spectrometers Surface wetting and contact angles Future trends References

26 27 44 50 53 54

v

vi

3

Contents

Textile surface functionalization by physical vapor deposition (PVD)

58

Q. WEI, Y. XU AND Y. WANG, Jiangnan University, China

3.1 3.2 3.3 3.4 3.5 3.6 4

Introduction Working principles of physical vapor deposition Funtionalization of textiles by sputtering Interfacial bonding Future trends References

58 59 63 79 86 86

Surface grafting of textiles

91

N. ABIDI, Texas Tech University, USA

4.1 4.2 4.3 4.4 4.5 4.6 4.7 5

Introduction Techniques of surface grafting Properties achieved and applications Strengths and weaknesses of surface grafting Future trends Sources of further information and advice References

91 91 101 104 105 105 105

Modification of textile surfaces using electroless deposition

108

S. Q. JIANG AND R. H. GUO, The Hong Kong Polytechnic University, China

5.1 5.2 5.3 5.4 5.5 5.6 5.7

Introduction The techniques and key principles of electroless deposition Characterisation of electroless copper- and nickel-plated polyester fabrics Strengths and weaknesses of electroless deposition Future trends Sources of further information and advice References

108 109 112 122 123 123 124

Contents

6

Textile surface functionalisation by chemical vapour deposition (CVD)

vii

126

J. I. B. WILSON, Heriot-Watt University, UK

6.1 6.2 6.3 6.4 6.5 6.6 6.7 7

Introduction Practical methods for chemical vapour deposition Characteristics of chemical vapour deposition coatings Applications Current trends and potential advances in uses and techniques Further sources of information References

126 129 132 133 136 137 137

Enzyme surface modification of textiles

139

V. A. NIERSTRASZ, Ghent University, Belgium

7.1 7.2 7.3 7.4 7.5 7.6 8

Introduction: principles of enzyme surface modification of textile materials Enzymes, technologies and materials (natural materials, synthetic materials, biomaterials) Strengths and weaknesses of enzyme surface modification Future trends Acknowledgements References

142 155 156 157 158

Modification of textile surfaces using nanoparticles

164

139

N. VIGNESHWARAN, Central Institute for Research on Cotton Technology, India

8.1 8.2 8.3 8.4 8.5 8.6 8.7

Introduction Nanoparticles synthesis and characterization Functional properties using nanoparticles Commercialization of nanofinishing in textiles Strengths and weaknesses of nanotechnology for surface modification Future trends References

164 165 169 180 182 182 182

viii

9

Contents

Modification of textile surfaces using the sol-gel technique

185

T. TEXTOR, Deutsches Textilforschungszentrum Nord-West e.V., Germany

9.1 9.2 9.3 9.4 9.5

Introduction: the principles of the sol-gel technique General aspects of textile finishing using (nano-)sols Sol-gel-based finishing effects Future trends References

185 190 197 209 210

10

Nano-modification of textile surfaces using layer-by-layer deposition methods

214

P. LU, University of California, USA; and B. DING, Donghua University, China

10.1 10.2 10.3 10.4 10.5 10.6

Introduction The LbL deposition technique LbL deposition on textile surfaces Conclusions and future trends Acknowledgements References

214 218 221 232 233 233

11

Surface modification of textiles for composite and filtration applications 238 A. S. HOCKENBERGER, Uludag University, Turkey

11.1 11.2 11.3 11.4 11.5 11.6 11.7

Introduction Surface modification of textiles for composites Surface properties of reinforcing fibers and applications Surface modification of textiles for filtration Applications of surface-modified fibers used for filtration Future trends References

238 239 246 257 260 262 264

Contents

12

Surface modification of textiles by aqueous solutions

ix

269

J. WANG, The Procter & Gamble Company, USA; and J. LIU, Zhejiang Sci-Tech University, China

12.1 12.2 12.3 12.4 12.5 13

Introduction Mechanisms and chemistries of textile surface modifications Applications of surface modification of textiles by aqueous solutions Future trends References Surface modification of textiles by plasma treatments

269 270 280 291 293

296

R. R. MATHER, Heriot-Watt University, UK

13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10 13.11

Introduction Nature of plasmas Plasma generation Low-pressure versus atmospheric-pressure treatments Strengths and limitations of plasma treatments Characterisation of plasma-treated textile surfaces Modifications to textile surfaces Applications Future trends Sources of further information and advice References

296 297 297 299 300 301 303 311 313 314 314

14

Emerging approaches to the surface modification of textiles

318

Q. WEI, Jiangnan University, China

14.1 14.2 14.3 14.4

The expansion of textiles into technical applications New techniques for surface modification Future trends References

318 319 321 322

Index

325

Contributor contact details

(* = main contact)

Chapter 1

Chapter 4

Dr M. J. John and Dr R. D. Anandjiwala* CSIR Materials Science and Manufacturing Fibres and Textiles Competence Area P.O. Box 1124 Port Elizabeth 6000 South Africa

Dr N. Abidi Fiber and Biopolymer Research Institute Department of Plant and Soil Science Texas Tech University 1001 East Loop 289 Lubbock, TX 79403 USA

E-mail: [email protected]

Chapter 5

Chapter 2 Dr Q. Wei,* F. Huang and Y. Cai Key Laboratory of Eco-textiles Ministry of Education Jiangnan University Wuxi 214122 People’s Republic of China E-mail: [email protected]

Chapter 3 Dr Q. Wei,* Y. Xu and Y. Wang Key Laboratory of Eco-textiles Ministry of Education Jiangnan University Wuxi 214122 People’s Republic of China E-mail: [email protected]

E-mail: [email protected]

Dr S. Q. Jiang* and R. H. Guo Institute of Textiles and Clothing The Hong Kong Polytechnic University Hung Hom, Kowloon Hong Kong People’s Republic of China E-mail: [email protected]

Chapter 6 Professor J. I. B. Wilson Physics Department School of Engineering and Physical Sciences Heriot-Watt University Riccarton Edinburgh EH14 4AS Scotland UK E-mail: [email protected] xi

xii

Contributor contact details

Chapter 7

USA

Dr ir. V. A. Nierstrasz Department of Textiles Ghent University Technologiepark 907 9052 Gent/Zwijnaarde Belgium

E-mail: [email protected]

E-mail: [email protected]

Chapter 8

Dr B. Ding Modern Textile Institute Donghua University Shanghai 200051 People’s Republic of China E-mail: [email protected]

Chapter 11

Dr N. Vigneshwaran The Nanotechnology Research Group Central Institute for Research on Cotton Technology Adenwala Road, Matunga Mumbai 400019 India

Professor A. S. Hockenberger Uludag University Faculty of Engineering and Architecture Textile Engineering Department 16059 Gorukle-Bursa Turkey

E-mail: [email protected]

E-mail: [email protected]

Chapter 9

Chapter 12

Dr T. Textor Deutsches Textilforschungszentrum Nord-West e.V. Institut an der Universität DuisburgEssen Adlerstr. 1 47798 Krefeld Germany

Dr Jiping Wang* The Procter & Gamble Company 5289 Vine Street Cincinnati, OH 45217 USA

E-mail: [email protected]

Chapter 10 Dr P. Lu* Fiber and Polymer Science University of California Davis, CA 95616

E-mail: [email protected] Professor Jinqiang Liu Zhejiang Sci-Tech University #2 Street Xiasha Higher Education Zone Hangzhou Zhejiang 310018 People’s Republic of China E-mail: [email protected]

Contributor contact details

Chapter 13

Chapter 14

Dr Robert R. Mather School of Engineering and Physical Sciences Heriot-Watt University Riccarton Edinburgh EH14 4AS Scotland, UK

Dr Q. Wei Key Laboratory of Eco-textiles Ministry of Education Jiangnan University Wuxi 214122 People’s Republic of China

E-mail: [email protected]

E-mail: [email protected]

xiii

Woodhead Publishing in Textiles

1 Watson’s textile design and colour Seventh edition Edited by Z. Grosicki 2 Watson’s advanced textile design Edited by Z. Grosicki 3 Weaving Second edition P. R. Lord and M. H. Mohamed 4 Handbook of textile fibres Vol 1: Natural fibres J. Gordon Cook 5 Handbook of textile fibres Vol 2: Man-made fibres J. Gordon Cook 6 Recycling textile and plastic waste Edited by A. R. Horrocks 7 New fibers Second edition T. Hongu and G. O. Phillips 8 Atlas of fibre fracture and damage to textiles Second edition J. W. S. Hearle, B. Lomas and W. D. Cooke 9 Ecotextile ’98 Edited by A. R. Horrocks 10 Physical testing of textiles B. P. Saville 11 Geometric symmetry in patterns and tilings C. E. Horne 12 Handbook of technical textiles Edited by A. R. Horrocks and S. C. Anand 13 Textiles in automotive engineering W. Fung and J. M. Hardcastle 14 Handbook of textile design J. Wilson 15 High-performance fibres Edited by J. W. S. Hearle

Woodhead Publishing in Textiles 16 Knitting technology Third edition D. J. Spencer 17 Medical textiles Edited by S. C. Anand 18 Regenerated cellulose fibres Edited by C. Woodings 19 Silk, mohair, cashmere and other luxury fibres Edited by R. R. Franck 20 Smart fibres, fabrics and clothing Edited by X. M. Tao 21 Yarn texturing technology J. W. S. Hearle, L. Hollick and D. K. Wilson 22 Encyclopedia of textile finishing H. K. Rouette 23 Coated and laminated textiles W. Fung 24 Fancy yarns R. H. Gong and R. M. Wright 25 Wool: Science and technology Edited by W. S. Simpson and G. Crawshaw 26 Dictionary of textile finishing H. K. Rouette 27 Environmental impact of textiles K. Slater 28 Handbook of yarn production P. R. Lord 29 Textile processing with enzymes Edited by A. Cavaco-Paulo and G. Gübitz 30 The China and Hong Kong denim industry Y. Li, L. Yao and K. W. Yeung 31 The World Trade Organization and international denim trading Y. Li, Y. Shen, L. Yao and E. Newton 32 Chemical finishing of textiles W. D. Schindler and P. J. Hauser 33 Clothing appearance and fit J. Fan, W. Yu and L. Hunter 34 Handbook of fibre rope technology H. A. McKenna, J. W. S. Hearle and N. O’Hear 35 Structure and mechanics of woven fabrics J. Hu 36 Synthetic fibres: nylon, polyester, acrylic, polyolefin Edited by J. E. McIntyre 37 Woollen and worsted woven fabric design

xv

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Woodhead Publishing in Textiles

E. G. Gilligan 38 Analytical electrochemistry in textiles P. Westbroek, G. Priniotakis and P. Kiekens 39 Bast and other plant fibres R. R. Franck 40 Chemical testing of textiles Edited by Q. Fan 41 Design and manufacture of textile composites Edited by A. C. Long 42 Effect of mechanical and physical properties on fabric hand Edited by Hassan M. Behery 43 New millennium fibers T. Hongu, M. Takigami and G. O. Phillips 44 Textiles for protection Edited by R. A. Scott 45 Textiles in sport Edited by R. Shishoo 46 Wearable electronics and photonics Edited by X. M. Tao 47 Biodegradable and sustainable fibres Edited by R. S. Blackburn 48 Medical textiles and biomaterials for healthcare Edited by S. C. Anand, M. Miraftab, S. Rajendran and J. F. Kennedy 49 Total colour management in textiles Edited by J. Xin 50 Recycling in textiles Edited by Y. Wang 51 Clothing biosensory engineering Y. Li and A. S. W. Wong 52 Biomechanical engineering of textiles and clothing Edited by Y. Li and D. X. Q. Dai 53 Digital printing of textiles Edited by H. Ujiie 54 Intelligent textiles and clothing Edited by H. Mattila 55 Innovation and technology of women’s intimate apparel W. Yu, J. Fan, S. C. Harlock and S. P. Ng 56 Thermal and moisture transport in fibrous materials Edited by N. Pan and P. Gibson 57 Geosynthetics in civil engineering Edited by R. W. Sarsby 58 Handbook of nonwovens Edited by S. Russell

Woodhead Publishing in Textiles

xvii

59 Cotton: Science and technology Edited by S. Gordon and Y-L. Hsieh 60 Ecotextiles Edited by M. Miraftab and A. Horrocks 61 Composite forming technologies Edited by A. C. Long 62 Plasma technology for textiles Edited by R. Shishoo 63 Smart textiles for medicine and healthcare Edited by L. Van Langenhove 64 Sizing in clothing Edited by S. Ashdown 65 Shape memory polymers and textiles J. Hu 66 Environmental aspects of textile dyeing Edited by R. Christie 67 Nanofibers and nanotechnology in textiles Edited by P. Brown and K. Stevens 68 Physical properties of textile fibres Fourth edition W. E. Morton and J. W. S. Hearle 69 Advances in apparel production Edited by C. Fairhurst 70 Advances in fire retardant materials Edited by A. R. Horrocks and D. Price 71 Polyesters and polyamides Edited by B. L. Deopora, R. Alagirusamy, M. Joshi and B. S. Gupta 72 Advances in wool technology Edited by N. A. G. Johnson and I. Russell 73 Military textiles Edited by E. Wilusz 74 3D fibrous assemblies: Properties, applications and modelling of threedimensional textile structures J. Hu 75 Medical textiles 2007 Edited by J. Kennedy, A. Anand, M. Miraftab and S. Rajendran 76 Fabric testing Edited by J. Hu 77 Biologically inspired textiles Edited by A. Abbott and M. Ellison 78 Friction in textiles Edited by B .S. Gupta 79 Textile advances in the automotive industry Edited by R. Shishoo

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80 Structure and mechanics of textile fibre assemblies Edited by P. Schwartz 81 Engineering textiles: Integrating the design and manufacture of textile products Edited by Y. E. El-Mogahzy 82 Polyolefin fibres: industrial and medical applications Edited by S. C. O. Ugbolue 83 Smart clothes and wearable technology Edited by J. McCann and D. Bryson 84 Identification of textile fibres Edited by M. Houck 85 Advanced textiles for wound care Edited by S. Rajendran 86 Fatigue failure of textile fibres Edited by M. Miraftab 87 Advances in carpet technology Edited by K. Goswami 88 Handbook of textile fibre structure Edited by S. Eichhorn, J. W. S. Hearle, M. Jaffe and T. Kikutani 89 Advances in knitting technology Edited by T. Dias 90 Smart textile coatings and laminates Edited by W. C. Smith 91 Tensile failure of fibres handbook Edited by A. Bunsell 92 Interior textiles: Design and developments Edited by T. Rowe 93 Textiles for cold weather apparel Edited by J. Williams 94 Modelling and predicting textile behaviour Edited by X. Chen 95 Textiles for construction Edited by G. Pohl 96 Engineering apparel fabrics and garments J. Fan and L. Hunter 97 Surface modification of textiles Edited by Q. Wei

Preface

Textile industries have been experiencing fast development with versatile products for a wide spectrum of applications being developed through technological innovations. In various applications, the functions of textile materials are associated with phenomena such as wetting, biocompatibility, adsorption and electrical conductivity. Wetting, friction, adhesion, biocompatibility and adsorption all begin at the surface. In these cases, it is the surfaces, rather than the bulk compositions, that are critical to the material performance. The properties of textile surfaces depend closely on their surface chemical and physical structures, which vary according to differences in polymer composition, structures of fibers and fiber assemblies as well as treatment. The surfaces of textiles offer a platform for functional modifications to meet specific requirements for a variety of applications. Surface modification of textiles refers to the use of a wide range of technologies designed to modify the surface properties of textiles to create the surface structures that give the textile product the desired properties. The surface modification of textiles may be achieved by various techniques ranging from traditional solution treatments to biological approaches. Functionalization of textile surfaces has attracted a great deal of attention in recent years, with the help of new technologies such as high energy beam processes, vapor deposition and nanoparticle coatings. In textiles, the fiber is the basis for creating structural forms and functional properties, and by manipulating the bulk and surface structures of fibers, the resulting textiles may possess new capabilities and functions to meet the demands for particular uses. It has been recognized that surface analysis is an essential process in improving our understanding of surface structure and its relationship to the technical performance of textiles and the technologies used for the surface modification of textiles. New developments in surface analysis technologies allow surface characterizations to be achieved at the nano-scale level. This book provides a detailed review of the surfaces of textiles, the surface xix

xx

Preface

characterization and modification of textiles, and the applications of the modified textiles. It is believed that this book will promote understanding of surface modification and the development of surface modification strategies for various applications of textiles. Q. Wei

1 Surface modification and preparation techniques for textile materials M . J . J O H N and R . D . A N A N D J I W A L A

CSIR, South Africa

Abstract: This chapter presents an overview of some important surface modification techniques employed for improved functional behaviour of textiles. Textile materials are used in a variety of applications where surface modification is of profound importance as it improves various properties – such as softness, dyeability, absorbance and wettability. In this chapter, the most commonly used surface modification techniques, ranging from plasma treatment to nanocoatings, for both natural and synthetic fibres have been discussed. Recent studies involving the modification and characterisation of textiles have also been highlighted. Key words: textile, modification, preparation techniques, wet processing.

1.1

Introduction

Textile technology deals with several disciplines including: the structure, properties and behaviour of fibres; how fibres are assembled into fibrous structures and fabrics; surface modification of fibres; and the making, analysis, sale and end uses of fibres and fabrics. Of these disciplines, surface modification of textiles is of profound importance as it improves properties such as softness, adhesion and wettability. Functional properties can also be imparted to textile fibres. Textiles find use in a variety of applications, the most common of which are clothing, carpeting and furnishing. Textiles used for industrial purposes, and chosen for characteristics other than their appearance, are commonly referred to as technical textiles. Technical textiles include textile structures for automotive applications, medical textiles (e.g. bandages, pressure garments and implants), geotextiles (for reinforcement of embankments), agro-textiles (textiles for crop protection), protective clothing (e.g. against heat and radiation for fire-fighter clothing, against molten metals for welders, stab-proof clothing and bullet-proof vests). In all these applications, stringent performance requirements must be met. Textile fibres are classified into two main groups, i.e. natural and man-made, depending upon their origin. Natural fibres can be mainly divided into protein fibres of animal origin (wool, silk), plant fibres of cellulosic origin and mineral fibres. Man-made fibres include three main categories: 1

2

Surface modification of textiles Textile fibres

Man-made (see Fig. 1.2)

Natural

Plant origin

Leaf (sisal)

Bast (flax)

Animal origin

Mineral origin (asbestos)

Fruit (coir)

Silk

Wool

Hair

1.1 Classification of textile fibres.1

• from synthetic polymers, such as polyester, nylon and acrylic; • from regenerated cellulose, such as viscose and lyocell; • cellulose acetates, such as diacetate and triacetate. A number of reports on the classification of textile fibres have been published.1–4 There are also newly synthesised fibres engineered for high-performance end uses, for example, aramid and polysulfide fibres. Figures 1.1 and 1.2 give the detailed classification of textile fibres.

1.2

Natural fibres

1.2.1

Plant origin

Plant fibres include bast (or stem or soft sclerenchyma) fibres, leaf or hard fibres, seed, fruit, wood, cereal straw, and other grass fibres. Plant fibres are composed of cellulose, hemicelluose and lignin. Common examples of plant fibres are flax (bast fibre), sisal (leaf fibre) and cotton and oil palm (seed). Table 1.1 presents a list of

Surface modification and preparation techniques

3

Man-made fibres

Natural polymer

Regenerated protein (casein)

Synthetic polymer

Regenerated cellulose

Synthetic polymer, natural origin (PLA fibre)

Polyesters Polyamides Polyurethanes

Cellulose esters

Polyvinyl derivatives

Polymerised hydrocarbons

Polysulphides

Polybenzimidazoles

1.2 Classification of man-made fibres.1 (PLA = poly-lactic acid.)

commonly used plant fibres and their origin. The use of such plant fibres in reinforced composites has increased owing to their low costs, biodegradability and the fact that they can compete well with other fibres in terms of strength per weight of material.

1.2.2

Animal origin

Animal fibres include silk, wool and hair. Silk is the secretion product from silkworm. Wool is classified in the soft hair group while the coarse hair group comprises horse hair, cow hair and human hair. Animal fibres are composed mainly of proteins. Wool fibres are composed of keratin, a complex mixture of proteins characterised by the presence of a considerable amount of the amino acid cystine. The disulfide bond in this amino acid residue forms cross-linkages between different protein chains thereby making it more stable and less soluble than other proteins.5 Table 1.2 presents a list of commonly used animal fibres. Deer hair and racoon dog hair are also used, mainly for the bristles of paint brushes.

4

Surface modification of textiles

Table 1.1 List of important plant fibres Fibre source

Species

Origin

Abaca Bagasse Bamboo Banana Broom root Cantala Caroa China jute Coir Cotton Curaua Date palm Flax Hemp Henequen Isora Istle Jute Kapok Kenaf Kudzu Mauritius hemp Nettle Oil palm Phormium Piassava Pineapple Roselle Ramie Sansevieria (Bowstring hemp) Sisal Sponge gourd Straw (cereal) Sun hemp Cadillo/urena Wood

Musa textilis – (>1250 species) Musa indica Muhlenbergia macroura Agave cantala Neoglaziovia variegate Abutilon theophrasti Cocos nucifera Gossypium sp. Ananas erectifolius Phoenix dactylifera Linum usitatissimum Cannabis sativa Agave fourcroydes Helicteres isora Samuela carnerosana Corchorus capsularis Ceiba pentranda Hibiscus cannabinus Pueraria thunbergiana Furcraea gigantea Urtica dioica Elaeis guineensis Phormium tenas Attalea funifera Ananus comosus Hibiscus sabdariffa Boehmeria nivea Sansevieria

Leaf Grass Grass Leaf Root Leaf Leaf Stem Fruit Seed Leaf Leaf Stem Stem Leaf Stem Leaf Stem Fruit Stem Stem Leaf Stem Fruit Leaf Leaf Leaf Stem Stem Leaf

Agave sisilana Luffa cylinderica – Crorolaria juncea Urena lobata (>10 000 species)

Leaf Fruit Stalk Stem Stem Stem

1.3

Synthetic fibres

Synthetic fibres form an important part of the textile industry, with the production of polyester alone surpassing that of cotton. There are many different kinds of synthetic fibres but among them polyamide is widely used, for example nylon. The fibres of polyvinyl alcohol and polypropylene (PP) are also important. Given this importance, research into effective production of these fibres and improving fibre

Surface modification and preparation techniques

5

Table 1.2 List of important animal fibres Animal fibre

Origin

Silk (domestic silk, wild silk) Wool Cashmere wool Camel wool Mohair Alpaca wool Rabbit hair Cow hair

Silkworm Sheep Goat reared in Himalayan region Camel Angora goat Goat reared in Andes mountains in Peru Angora rabbit Cow

From reference 6.

properties is warranted. A great disadvantage of some synthetic fibres is their low hydrophilicity. This affects the processing of the fibres especially during wet treatments. The surfaces are not easily wetted, thus impeding the application of finishing compounds and colouring agents. In addition, a hydrophobic material hinders water from penetrating into the pores of fabric. Nonwoven fabrics are formed by extrusion processes and may be manufactured inexpensively so that they can be used in disposable products that are discarded after only one or a few uses. For example, polypropylene (PP) and polyester nonwoven fabrics are used in disposable absorbent articles, such as diapers, feminine care products and wipes. Moreover, they are also widely used as filtration media, battery separators and geotextiles. In all these applications, the PP and polyester nonwoven fabrics need to be wettable by water or aqueous liquids, which is not an inherent characteristic of the material. Surface modification of textiles is performed to improve various properties such as softness, dyeability, absorbance and wettability. Recent advances in textile chemistry have resulted in imparting various functional properties such as antimicrobial activity, decreased skin irritation properties and also enhanced fragrance to textiles.

1.4

Surface preparation techniques for textile materials

Surface preparation techniques are mainly used for the removal of foreign materials to improve uniformity, hydrophilic nature and affinity for dyestuffs and other treatments and relaxation of residual tensions in synthetic fibres. The surface preparation technique usually depends on the type of fibre (natural or synthetic) and the form of the fibrous structure (i.e. spun yarn, woven or knitted fabric). Some of the common preparation techniques are outlined below.

6

1.4.1

Surface modification of textiles

Singeing

Singeing is usually carried out on woven cotton fabrics and yarns to burn protruding fibres which affect subsequent processing, such as dyeing and finishing. The fabric is passed over a row of gas flames and then immediately dipped into a quench bath to extinguish the sparks and cool the fabric. The quench bath often contains a desizing solution, in which case the final step in singeing becomes a combined singeing and desizing operation.

1.4.2

Desizing

Desizing is used for removing previously applied sizing compounds from woven fabric and is usually the first wet finishing operation performed on woven fabric. Different desizing techniques are employed depending upon the kind of sizing agent to be removed. Sizing agents are commonly based on natural polysaccharides (starch, protein and cellulosic derivatives) and synthetic polymers (polyvinyl alcohol (PVA), polyacrylates, polyesters and polyvinyl acetates). Depending on the sizing agent used, sizes can be removed using hot water, enzymes or hydrogen peroxide. Currently applied techniques can be categorised as follows: (a) removal of starch-based sizing agents (water-insoluble sizes); (b) removal of water-soluble sizes (PVA and polyacrylates).

1.4.3

Scouring

The purpose of scouring is to extract impurities present in the raw fibre or picked up at a later stage – such as pectins, fat and waxes, proteins, inorganic substances (e.g. alkali metal salts), calcium and magnesium phosphates, aluminium and iron oxides, sizes (when scouring is carried out on woven fabric as part of desizing), residual sizes and sizing degradation products (when scouring is carried out on woven fabric after desizing). Scouring can be carried out as a separate step of the process or in combination with other treatments (usually bleaching or desizing) on all kind of substrates: woven fabric (sized or desized), knitted fabric and yarn. The action of scouring is performed by an alkali (sodium hydroxide or sodium carbonate) together with auxiliaries that include non-ionic (alcohol ethoxylates, alkyl phenol ethoxylates) and anionic (alkyl sulfonates, phosphates, carboxylates) additives.

1.4.4

Bleaching

Bleaching is used to remove colour impurities in natural and some man-made fibres to produce a whiter substrate. This is usually accomplished by oxidising the natural pigments of the fibre using an oxidising agent, for example, hydrogen

Surface modification and preparation techniques

7

peroxide. Cotton is usually bleached using hydrogen peroxide under alkaline conditions, with the addition of sodium silicate and magnesium sulfate to stabilise the process and reduce fibre damage. Metal ions, such as iron and copper, can catalyse the decomposition of the hydrogen peroxide resulting in the formation of oxycellulose and localised damage to the fibres. Bleaching of cotton fabrics can be undertaken as continuous or pad batch processes. Newer research in this field is directed towards non-peroxide bleaching to reduce the adverse effects on the environment caused by effluent discharge. Non-peroxide bleaching agents like sodium hypochlorite and sodium chlorite are widely used but have lost favour because of environmental issues.

1.4.5

Mercerisation

Mercerisation is another technique and is used for cellulosic and cotton fibres in particular.7 The process involves the treatment of fabrics with sodium hydroxide and is named after John Mercer. It has been observed that compared with untreated cotton mercerised cotton has greater strength and lustre, is more absorbent and has a greater capacity to absorb dye. When natural fibres are treated with sodium hydroxide, it results in dissolution of hemicellulose and rearrangement of microfibrils in a more compact manner. Surface preparation techniques for man-made composites would involve techniques like solvent cleaning with solvents such as isopropyl alcohol, surface abrasion, and conditioning and neutralisation with suitable chemical agents.

1.5

Surface modification techniques for textile materials

There are several surface modification techniques that have been developed to improve wetting, adhesion and other properties of textile surfaces by introducing a variety of reactive groups. Some of the common techniques are described below.

1.5.1

Wet chemical processing

In wet chemical surface modification, the textile surface is treated with liquid reagents to generate reactive functional groups on the surface. This technique results in the penetration of the textile substrate by the chemical agent. The commonly used chemical processing agents are chromic acid and potassium permanganate which introduce oxygen-containing moeties to PP and polyethylene fibres. The degree of surface functionalisation is therefore not repeatable between polymers of different molecular weight and crystallinity. This type of treatment can also lead to the generation of hazardous chemical waste and can result in surface etching.

8

1.5.2

Surface modification of textiles

Ionised gas treatments

Plasma treatment Plasma is a high-energy state of matter in which a gas is partially ionised into charged particles, electrons and neutral molecules. Gas plasmas were introduced in the 1960s but it is only recently that it has been possible to treat textiles on a commercial scale. Plasma is essentially a dry process providing modification of the top nanometre surface without using solvents or generating chemical waste. The type of functionalisation imparted can be varied by the plasma gas selected (e.g. Ar, He, N2, O2, H2O, CO2 and NH3) and operating parameters (e.g. pressure, power, time and gas flow rate). Oxygen plasma is used to impart oxygencontaining functional groups to polymer surfaces. Carbon dioxide plasma has been used to introduce carboxyl groups. Inert gases are used to introduce radical sites on the polymer surface for subsequent graft polymerisation. Exposure to plasma results in the cleaning of the surface of materials and modification of surface energies. Active species from the plasma bombard or react with monolayers on the surface of materials and change the properties either temporarily or permanently. The advantages of plasma technology are its potential environmental friendliness and energy conservation benefits in developing highperformance materials. The properties of natural and synthetic fibres are modified by processes such as polymerisation, grafting and cross-linking. As adhesion is a surface-dependent property, plasma technology can achieve modification of the near-surface region effectively without affecting the bulk properties of the materials of interest. The common benefits of plasma surface modifications on textile materials are enhancements in wettability8, 9, printability10, 11, adhesion 12, 13 and sterilisation.14 Plasma can also be used as a precursor to other surface modification techniques; for example, plasma activation followed by ultraviolet (UV) graft polymerisation or plasma activation followed by silane treatment. Dielectric barrier discharges (DBDs) or ‘silent’ discharges are widely used for the plasma treatment of polymer films and textiles.15, 16 These discharges demonstrate great flexibility with respect to their geometric shape, working gas composition and operational parameters (input power, frequency of the applied voltage, pressure, gas flow, substrate exposure time, etc.). A DBD is obtained between two electrodes, at least one of which is covered with a dielectric, when a high-voltage alternating current is applied between the electrodes. The most interesting property of DBDs is that in most gases the breakdown starts at many points, followed by the development of independent current filaments (termed ‘micro-discharges’). These micro-discharges are of nanosecond duration and are uniformly distributed over the dielectric surface. A disadvantage of plasma treatment is that plasma generation requires a vacuum to empty the chamber of latent gases; this is complicated for continuous operation

Surface modification and preparation techniques

9

in a large-scale industrial setting. In addition, there are several processing parameters to optimise – including time, temperature, power and distance of substrate from plasma source – which can affect the reproducibility of results. Morent et al.17 have presented an interesting overview of the literature on the treatment of textiles with non-thermal plasmas. Corona discharge Corona discharge is a low-cost, simple process in which an electrically induced stream of ionised air bombards the polymer surface. It is usually used to promote adhesion in inert polymers. A disadvantage of the process is contamination of the polymer surface since vacuum conditions are not required. Flame treatment Flame treatment is a non-specific surface functionalisation technique that bombards the polymer surface with ionised air generating large amounts of surface oxidation products. The reactive oxygen is generated by burning an oxygen-rich gas mixture. Flame treatment has been shown to impart hydroxyl, aldehyde and carboxylic acid functionalities to polyethylene and is utilised to enhance printability, wettability and adhesion. One drawback of flame treatment is that it can reduce the optical clarity of polymers; in addition, there are many parameters (including flame temperature, contact time and composition) that must be accurately controlled to maintain consistent treatment and to avoid burning. Ultraviolet irradiation When polymer surfaces are exposed to UV light, polymer surfaces generate reactive sites which can become functional groups upon exposure. The difference between ionised gas treatments and UV irradiation is the ability to tailor the depth of surface reactivity by varying wavelength and absorption coefficient.

1.6

Recent studies on the modification of textiles

Strnad et al.18 investigated the effect of chitosan treatment on sorption and mechanical properties of cotton fibres. Cotton fibres initially underwent different pre-treatments (alkali treatment, bleaching, demineralisation) and were then oxidised using differing procedures containing KIO4 solutions and then treated with chitosan. The authors observed that all the oxidation procedures, even under very mild conditions, had a significant influence on the worsening of mechanical properties. The treatment of oxidised cotton with chitosan had no influence on breaking force and elongation, but increased the Young’s modulus of fibres. This was attributed to the fact that chitosan bound itself to raw cotton through inter-

10

Surface modification of textiles

(a)

4µm

9.3KV

02

172

S

(b)

1.3 Surface structure of polyester fibre (a) untreated (b) under high fluence (5 pulses at 100 mJ/cm2) laser irradiation.19

Surface modification and preparation techniques

11

fibrillar linkages with the primary walls of the fibres. Chitosan adsorption was also found to increase the moisture absorption of the fibres. The effect of laser modification on the properties of polyester was investigated by Kan.19 The properties studied included tensile strength, elongation, wetting and crystallinity. The author observed that laser irradiation did not affect the bulk properties owing to its low penetration depth. Morphological study of the untreated and treated polyester fibres revealed a ripple-like structure on the treated fibres. Figures 1.3(a) and (b) show the presence of a smooth surface while laser irradiation resulted in a roll-like to ripple-like structure. The orientation of this kind of ripple-like structure was found to be perpendicular to the fibrillar orientation of the fibre. The tensile strength and elongation were also found to decrease after irradiation. This was attributed to the fact that ripples created more weak points in the fibre leading to a reduction in tensile strength. In a recent study, by Morent et al.20 polyethylene terephthalate (PET) and PP nonwovens were modified by a dielectric barrier discharge in air, helium and argon at medium pressure (5.0 kPa). The helium and argon discharges contained an air fraction smaller than 0.1%. Surface analysis and characterisation were performed using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS measurements revealed the incorporation of oxygen-containing functional groups (C–O, C=O and O–C=O) on the PP and PET nonwovens. It was also observed that air plasma was more efficient at incorporating oxygen functionalities than argon plasma, which was in turn more efficient than helium plasma. Figures 1.4(a) and (d) show SEM images of untreated PP and PET nonwovens, respectively. The untreated PP nonwoven consisted of PP fibres with a relatively smooth surface; the surface of the PET fibres was also relatively smooth, however small particles were present on the fibre surfaces. Energy-dispersive X-ray microanalysis (EDX) identified the particles as NaCl crystals. The presence of these crystals was due to the insufficient removal of chemicals during the production process of the nonwoven. Figures 1.4(b) and (e) show SEM images of the saturated PP and PET nonwovens, respectively, after plasma treatment in air. The SEM images show that the saturated PP and PET nonwovens had the same surface morphology as the untreated samples. Figures 1.4(c) and (f) show SEM images of the PP and PET nonwovens, respectively, after plasma treatment in air with a very high energy density (1.13 J/cm2). After extended plasma treatment, the surfaces of the PP and PET fibres showed a significant accumulation of PP and PET matter, respectively. Borcia et al.21 investigated the effects of plasma treatment of woven natural, synthetic and mixed fabrics, using a DBD run in various environments (air, argon and nitrogen) at atmospheric pressure. The experiments were conducted to determine the effects of the operating parameters (dielectric layer make-up, discharge energy density, gas flow, gas type and exposure time) on the measured changes to surface wettability, morphology and chemical composition. It was

12

Surface modification of textiles

(a)

(d)

(b)

(e)

(c)

(f)

1.4 SEM images of (a) the untreated PP non-woven, (b) the PP nonwoven after plasma treatment in air (energy density = 420 mJ/cm2), (c) the PP non-woven after plasma treatment in air (energy density = 1.13 J/cm2) , d) the untreated PET non-woven, (e) the PET non-woven after plasma treatment in air (energy density = 230 mJ/cm2) and (f) the PET non-woven after plasma treatment in air (energy density = 1.13 J/cm2).20

observed that surface modification of woven natural, synthetic and mixed fabrics showed that a DBD plasma can be used to modify the surface of textured textile materials, leading to enhanced hydrophilicity-dependent properties. The

Surface modification and preparation techniques (a)

13

C1

C2

Intensity

C3

(b)

290

288

286

284

282

Binding energy (eV)

1.5 Carbon (1s) XPS spectra for (a) untreated and (b) 1.0 s-treated polyester fabric.21

behaviour of the woven textile polymers was found to be very similar, under DBD treatment, to that of thin-film variants of the same polymers. The surface properties, such as the wickability and the level of oxidation, were increased markedly after the treatment. Figure 1.5 shows the XPS spectra of untreated and treated polyester fabrics. The XPS spectra of untreated samples include high binding energy carbon peaks, as shown in Fig. 1.5(a) for the polyester fabric. The C1s spectrum for untreated polyester consists mainly of three distinct peaks: the hydrocarbon atoms C1 (285.0 eV), the methylene carbon singly bonded to oxygen C2 (286.7 eV) and the ester carbon atoms C3 (288.9 eV). Although the same chemical functionalities can be identified on the treated samples (Fig. 1.5(b)), the oxidation is demonstrated by the increase in intensity of the C2 and C3 peaks as compared with the C1 peak. Masaeli et al.22 investigated the effect of treatment time with low-pressure oxygen plasma on the wettability, surface characteristics and fine structure of spunbonded PP nonwoven. It was observed that oxygen plasma treatment increased the surface wetting of PP fibres significantly. This was the result of an increase in surface free energy as well as the etching effect of the plasma. The wetting properties were measured using contact angle measurements. Table 1.3 compares the contact angles of water and methylene iodide on plasma-treated and

14

Surface modification of textiles

Table 1.3 Contact angle (degrees) of oxygen-plasma-treated and untreated samples22 Contact angle Sample

Water

Methylene iodide

Untreated Oxygen plasma treated (30 min, 500 W)

121.5 83.9

99.9 88.2

untreated nonwoven fabrics. As a result of plasma treatment, the contact angle decreased from 121.5º to 83.9º and from 99.9º to 88.2º for water and methylene iodide, respectively, indicating an increase in wettability. It was also noted that a longer plasma exposure time can produce more hydrophilic functional groups on the PP surface. The SEM images of the plasma-treated and untreated PP fibres are shown in Fig. 1.6(a) and (b). It is clear that the plasma treatment etched PP fibres significantly. The etching effect was caused by the bombardment of the fibre surface by active species in plasma radiation. XRD spectra of the samples (Fig. 1.7) revealed that plasma treatment had no significant effect on the crystallinity of the treated fibres. This was expected because the plasma action is limited to the surface of the fibres. Canal et al.23 examined the role of (N2, N2 + O2 and O2) post-discharge plasma modification on wool and polyamide fabrics. Dynamic contact angle, XPS and SEM were used to characterise the modified wool surfaces and revealed an improved wettability that was attributed to the generation of new chemical groups and the reduction/elimination of the fatty layer on the surface of the wool. Wool exhibits hydrophobic properties as a result of the presence of a thin lipid layer, called a ‘fatty layer’, on the outermost part of the fibres, surrounding each cuticle cell (scale). The fatty layer is formed by fatty acids which are covalently bonded to the protein matrix of the epicuticle (with a global thickness of 5–7 nm) by thioester links.24 The scales on the surface of wool fibres overlap one another like tiles on a roof, and are responsible for the directional frictional effect. Figure 1.8(a) shows the SEM micrograph of an untreated merino wool fibre, it can be observed that the cuticular cells are flat, the roughness of the wool fibre comes from the overlapping of the cells. A N2 post-discharge treatment did not produce remarkable changes on the surface of wool, except for the cleaning of the surface (Fig. 1.8(b)). In direct nitrogen plasma discharges there was no modification on the surface of wool, confirming the ineffectiveness of this kind of plasma gas treatment. In contrast, micrographs (Fig. 1.8(c)) of wool treated with oxygen post-discharge showed the presence of the visible etching effects through the presence of striations and micro-craters on the wool cuticle. The treatment of textile fabrics with ozone has been of interest in the field of textile finishing from the standpoint of environmental preservation.25, 26 Ozone bleaching of cotton fabric and ozone shrink-resistant finishing of wool fabric has

Surface modification and preparation techniques

15

(a)

(b)

1.6 SEM images of (a) untreated fibres and (b) fibres treated with oxygen plasma for 30 min.22

been reported.27, 28 Lee et al.29 investigated the ozone gas treatment of nylon 6 and polyester fabrics. The treatment incorporated much more oxygen into the fibre surface in the form of –OCOH and –OCOOH as shown by XPS. Water penetration increased considerably with treatment, and the apparent dyeing rate and

16

Surface modification of textiles 1200 Untreated Treated 1000

Counts

800

600

400

200

0 0

5

10

15

20

25

30

35

40

45

2θ (degrees)

1.7 XRD results of untreated samples and samples treated with oxygen plasma for 30 min.22

equilibrium dye uptake were also improved, especially for the polyester fabric, despite an increase in the crystallinity. Thus, it was observed that the treatment brought about a change not only in the fibre surface but also in the internal structure of the fibres (the crystalline and amorphous regions) with regard to the dyeing behaviour. Table 1.4 shows the relative intensities of the C1s, O1s and N1s spectra in the wide-scanning XPS of the ozone-gas-treated nylon 6 and polyester fabrics. As shown in Table 1.4, the O1s intensity of the nylon 6 fabric apparently increased when treated at atmospheric pressure, whereas the intensity of the polyester fabric increased with increasing gas pressure. An interesting approach to surface modification is the use of enzymes for this purpose.30–32 Examples of applicable enzymes are lipases and cutinases.33–35 They have been reported to increase hydrophilicity of polyesters by hydrolysis of ester bonds. Owing to the size of enzymes, they are only active at the surface so that the bulk characteristics of fibres remain unchanged. Reactions generally occur under mild conditions, no complicated machinery is required, as is the case for plasma treatments or etching procedures, and little or no additional chemicals are necessary. Cutinases and carboxylesterases have both shown the potential to hydrolyse ester bonds in a similar manner to lipases.36 The enzymatic surface modification of PET with cutinase from Fusarium solani pisi was investigated by

Surface modification and preparation techniques (a)

17

(b)

(c)

1.8 SEM micrographs of wool fibres: (a) untreated; (b) treated for 900 s in an N2 post-discharge; (c) treated for 900 s in an O2 post-discharge.23

Table 1.4 Relative intensities of C1s, O1s and N1s in wide-scanning XPS analysis of nylon 6 and polyester fabrics treated with ozone gas29 Surface chemical composition (%) Treatment Nylon 6 Untreated Ozone gas treated AP, 20 ºC/10 min Polyester fabric Untreated Ozone gas treated AP, 20 ºC/10 min 0.1 MPa, 20 ºC/10 min AP, atmospheric pressure.

C1s

O1s

N1s

83.5

12.7

3.8

79.8

15.5

4.7

75.2

24.8

–

74.8 74.2

25.2 24.8

– –

18

Surface modification of textiles

(a)

(b)

50 µm

(c)

50 µm

(d)

50 µm

50 µm

1.9 SEM micrographs of Scourzyme-treated hemp fibres.38

Vertommen et al.37 It was observed, by direct measurement and identification of the different products, that cutinase from F. solani pisi displayed significant hydrolytic activity towards amorphous regions of PET. In another interesting study, Ouajai and Shanks38 looked into the bioscouring of hemp fibre using pectate lyase (EC 4.2.2.2) (Trade name: Scourzyme L). Greater enzyme concentration and a longer treatment improved the removal of the low methoxy pectin component as indicated by UV spectroscopy. Removal of pectate caused no crystalline transformation in the fibres, except for a slight decline in the crystallinity index. Figure 1.9(a) shows fibre bundles of untreated hemp covered by non-cellulosic materials. The fibre bundles were 80–100 mm in diameter. Figure1.9(b) indicates that a treatment without the pectate lyase enzyme was not sufficient to remove all of the non-cellulosic materials from the fibres; fibres treated by buffer solution (pH 8.5) alone exhibited a considerable surface roughness. Only the fractions that were soluble in the buffer were partially extracted. The surface of 1.2% Scourzyme L-treated fibres (Fig. 1.9(c)) appears smooth, but the fibres are still assembled in bundles. A further elimination of pectin was exhibited after an extended treatment time of 6 h (Fig. 1.9(d)), resulting in a smoother surface and deeper inter-fibrillar disintegration of the bundle.

Surface modification and preparation techniques

19

The influence of a chitosan application on wool fabric before a treatment with a proteolytic enzyme was investigated by Vílchez et al.39 The enzymatic treatment enhances whiteness and confers shrink resistance to wool, but an increase in the enzyme concentration leads to a detrimental effect on the physico-mechanical properties. A chitosan treatment before the enzymatic treatment was also found to improve the shrink resistance and increase the weight loss. Ibrahim et al.40 attempted to enhance the antibacterial properties of cotton fabrics via pre-cross-linking with trimethylol melamine followed by subsequent treatment with iodine solution to create new active sites. The treated fabric showed the ability to inhibit as well as to arrest the growth of both Bacillus subtilis and Escherichia coli. The antibacterial activity is determined by the degree of modification and the extent of iodine retention, as well as the ease of iodine liberation. Another concept for the modification of synthetic and natural fibres is based on the permanent fixation of supramolecular components, e.g. cyclodextrins, on the surfaces by functional groups using common technologies of textile processing. An advantage of this method is that the textile fibres achieve specific properties by means of inclusion of inorganic molecules within the cyclodextrin cavities.41, 42 Cyclodextrins possess a hydrophobic cavity in which a number of chemicals can form inclusion complexes. They have been grafted to cellulosic and polyamide fabrics in order to investigate their use as textile finishing agents. Fourier transform infra red (FTIR) spectroscopy, SEM and absorbance spectra confirmed their immobilisation, and standard textile tests established that surface modification treatments did not affect mechanical integrity. An antimicrobial agent (benzoic acid), an odour-producing compound (vanillin), and an insect repellent pesticide (N, N-diethyl-meta-toluamide (DEET)) were some of the chemicals investigated for inclusion in these bound cyclodextrins.43–45 The unique ability of cyclodextrins to form reversible complexes with a range of chemicals makes them attractive in a number of polymer-bound applications, including drug delivery, odour removal and fragrance applications. In a study by Denter and Schollmeyer46 both synthetic and natural fibres were subjected to cyclodextrin fixation. It was reported that properties such as hydrophilicity, physiological and electrostatic power, reactivity and complexing power were significantly affected by ligand fixation. The surface modification of PP fibres by the addition of non-ionic melt additives (nonyl phenol ethoxylates and stearyl alcohol ethoxylates) has been reported by Vasantha et al.47 Melt additives can be blended with the polymers prior to or during melt spinning and they are bound in the polymer matrix when the polymer cools. The melt additive can migrate to the surface imparting durable hydrophilicity without altering the bulk properties of polymer. An interesting approach is the functionalisation of textile fibres by making use of concepts of nanotechnology.48 Coatings based on nanosols and inorganic– organic hybrid polymers, derived from the sol-gel process, have immense potential

20

Surface modification of textiles

for creative modifications of surface properties of textiles and can be applied with a comparatively low technical effort and at moderate temperatures. Coatings of a thickness of less than one micron can act as effective barriers against chemical attacks, super-repellent surfaces can be created, or the wear-resistance of textile materials improved. Coatings incorporating nanoparticles as employed in sun creams protect sensitive polymers against decomposition due to UV radiation. Ballistic bodywear based on fabrics with protection against gun attacks and fabrics that are resistant to knife cuts can be developed. For these products, thin coatings based on inorganic–organic hybrid polymers filled with alumina nanoparticles were found to give good stab-resistance. Further approaches deal with reversible photochromic coatings – coatings that change colour if irradiated with sunlight – magnetic hybrid polymers or medical systems based on porous sol-gel coatings with immobilised drugs that are released in contact with skin. In an interesting study, Becheri et al.49 reported on the synthesis and characterisation of nanosized ZnO particles and their application on cotton and wool fabrics for UV shielding. The effectiveness of the treatment was assessed through UV–visible light spectrophotometry and the calculation of the UV protection factor (UPF). The authors observed that the performance of ZnO nanoparticles as UV absorbers, can be efficiently transferred to fabric materials through the application of ZnO nanoparticles on the surface of cotton and wool fabrics. The UV tests indicate a significant increment in the UV absorbing activity in the ZnO-treated fabrics. Such results can be exploited for the protection of the body against solar radiation and for other technological applications. In a recent study by Leroux et al.50 a fluorocarbon nanocoating was deposited on polyester (PET) woven fabric using pulse discharge plasma treatment by injecting a fluoropolymer directly into the plasma DBD. The objective of the treatment was to improve the hydrophobic properties as well as the repellent behaviour of the polyester fabric. The study showed that adhesion of the fluoropolymer to the woven PET was greatly enhanced by the air plasma treatment. In another interesting study, Wei et al.51 investigated the functionalisation of PET by plasma modification. The surface topography and wetting characteristics were studied using environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). Figure 1.10(a) shows an ESEM photomicrograph of the untreated PET nonwoven material. The image reveals the fibrous structure of the material, but at this magnification the surface morphology of individual fibres in the web is not clear. The AFM image at higher magnification clearly indicates the surface characteristics of the untreated PET fibre in the web. As illustrated in Fig. 1.10(b), the surface of the PET fibre is quite smooth with some groove-like structures on the surface before plasma treatment. The groovelike surface is the fibril structure of the fibre. The wetting of the PET fibres observed in the ESEM image reveals that water droplets are formed on the fibre surface, which is shaped like the segments of spheres (as illustrated in Fig. 10(c)), and so the fibres are not properly wetted. This observation reflects the hydrophobic

Surface modification and preparation techniques

21

(b)

(a)

1

2

3

4

µm

X 1.000 µm/div Z 473.149 nm/div

(c)

1.10 Images of PET nonwoven and fibres: (a) ESEM image of PET nonwoven, (b) AFM image of the PET fibre surface and (c) wetting of water on the PET fibre surface.51

(a)

(b)

1 2 3

4

X 1.000 µm/div µm Z 150.000 nm/div

1.11 Activated PET nonwoven: (a) AFM image of the fibre surface and (b) ESEM image of the wetting of the fibres.51

22

Surface modification of textiles

behaviour of the PET fibres. However, after exposure to oxygen the fibre surface becomes pitted, as shown in Fig. 1.11(a). The surface of the fibre is roughened due to the etching effect of oxygen plasma. Oxygen plasma treatment also significantly affects the behaviour of the PET fibre surface towards water. The effects of plasma treatment are clearly discerned from the ESEM photomicrographs, as shown in Fig. 1.11(b). After oxygen plasma treatment, the profiles of the water droplets are considerably altered. The droplets are spread along the fibre surface, and the water droplets formed on the fibre surface show much lower contact angles, indicating the much better wetting properties of the treated fibres.

1.7

Future trends

A considerable amount of research is taking place in the field of surface modification of textiles with a view to improving properties and processes. In the case of conventional textile wet processes, the trend will be to reduce water and energy consumption and eliminate environmentally harmful effluent discharges resulting from the use of synthetic chemicals. Of the different kinds of surface modification techniques applied, plasma treatment appears to be the most commonly used and with good results and environmental benefits. Both equipment manufacturers and researchers are making concerted efforts to improve the commercial viability of plasma processes; however, doubts remain about the durability of the plasma treatments and their commercial viability. Enzymatic modification of textiles usually occurs at the surface of textiles owing to the large structures of enzymes and the use of enzymes will penetrate into different areas of the industry in the future. The use of naturally abundant substances, such as chitin and chitosan, will also be researched further. Nanotechnology offers promise owing to its potential to improve functional properties, for example super-hydrophobicity and hydrophilicity. Both intrinsic and surface modifications using nano-additives will be researched and the processes will be modified accordingly. The superhydrophobicity effect is seen in plants like lotus, taro and nasturtium and is termed the Lotus-Effect. The effect arises because lotus leaves have a very fine surface structure and are coated with hydrophobic wax crystals of around 1 nm in diameter. Surfaces that are rough on a nanoscale tend to be more hydrophobic than smooth surfaces because of the reduced contact area between the water and solid. In the lotus plant, the actual contact area is only 2–3% of the droplet-covered surface. The nanostructure is also essential to the self-cleaning effect – on a smooth hydrophobic surface, water droplets slide rather than roll and do not pick up dirt particles to the same extent. Researchers are currently using this technology in developing Lotus-Effect aerosol spray and clothing (http://nanotechweb.org/ cws/article/tech/16392). The ideal surface modification techniques will introduce a monolayer of a desired functional group without causing irregular etching or producing significant hazards.

Surface modification and preparation techniques

1.8

23

References

1 GOSWAMI B. C., ANANDJIWALA R. D. AND HALL D. M. (2004), Textile Sizing, Marcel Dekker Inc., New York. 2 COOK J. G. (1984), Handbook of Textile Fibres, Parts 1 and 11, 5th Edition, Merrow, Durham, UK. 3 MONCRIEF R. W. (1975), Man-made Fibres, 6th Edition, Wiley, New York. 4 MAUERSBERGER H. R. (Ed.) (1954), Mathews Textile Fibres, 6th Edition, John Wiley & Sons, New York. 5 SCHICK M. J. (1975), Surface Characteristics of Fibers and Textiles, Part 7, Marcel Dekker Inc., New York. 6 NAKAMURA A. (2000), Fiber Science and Technology, Science Publishers Inc., Enfield, USA. 7 HILL D. J., HALL M. E., HOLMES D. A., LOMAS M. AND PADMORE K. (1993), An Introduction to Textiles, Volume IV- Textile Wet Processing, Eurotex, Guimarães, Portugal. 8 DE GEYTER N., MORENT R. AND LEYS C. (2006), Surface modification of a polyester non-woven with a dielectric barrier discharge in air at medium pressure, Surf. Coat. Technol. 201(6), 2460–2466. 9 CHENG C., ZHANG L. Y. AND ZHAN R. J. (2006), Surface modification of polymer fibre by the new atmospheric pressure cold plasma jet, Surf. Coat. Technol. 200(24), 6659– 6665. 10 RADETIC M., JOCIC D., JOVANCIC P., TRAJKOVIC R. AND PETROVIC Z. L. (2000), The effect of low-temperature plasma pretreatment on wool printing, Text. Chem. Color. Am. Dyest. Rep. 32(4), 55–60. 11 RYU J., WAKIDA T. AND TAKAGISHI T. (1991), Effect of corona discharge on the surface of wool and its application to printing, Text. Res. J. 61(10), 595–601. 12 SHAKER M., KAMEL I., KO F. AND SONG J. W. (1996), Improvement of the interfacial adhesion between Kevlar fiber and resin by using RF plasma, J. Compos. Technol. Res. 18(4), 249–255. 13 SHENTON M. J., LOVELL-HOARE M. C. AND STEVENS G. C. (2001), Adhesion enhancement of polymer surfaces by atmospheric plasma treatment, J. Phys. D Appl. Phys. 34(18), 2754–2760. 14 HEISE M., NEFF W., FRANKEN O., MURANYI P. AND WUNDERLICH J. (2004), Sterilization of polymer foils with dielectric barrier discharges at atmospheric pressure, Plasma Polym. 9(1), 23–33. 15 DE GEYTER N., MORENT R. AND LEYS C. (2006), Surface modification of a polyester non-woven with a dielectric barrier discharge in air at medium pressure, Surf. Coat. Technol. 201(6), 2460–2466. 16 BORCIA G., ANDERSON C. A. AND BROWN N. M. D. (2003), Dielectric barrier discharge for surface treatment: application to selected polymers in film and fibre form, Plasma Sources Sci. Technol. 12(3), 335–344. 17 MORENT R., GEYTER N. D., VERSCHUREN J., CLERCKE K. D., KIEKENS P. AND LEYS C. (2008), Non-thermal plasma treatment of textiles, Surf. Coat. Technol. 202(14), 3427– 3449. 18 STRNAD S., SAUPER O., JAZBEC A. AND KLEINSCHEK K.S. (2008), Influence of chemical modification on sorption and mechanical properties of cotton fibers treated with chitosan, Text. Res. J. 78, 390–398.

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19 KAN C.W. (2008), Impact on textile properties of polyester with laser. Optics Laser Technol. 40, 113–119. 20 MORENT R., DE GEYTER N., LEUS C., GENGEMBRE L. AND PAYEN E. (2007), Surface modification of non-woven textiles using a dielectric barrier discharge operating in air, helium and argon at medium pressure, Text. Res. J. 77(7), 471–488. 21 BORCIA G., ANDERSON C. A. AND BROWN N. M. D. (2006), Surface treatment of natural and synthetic textiles using a dielectric barrier discharge, Surf. Coat. Technol. 201, 3074– 3081. 22 MASAELI E., MORSHED M. AND TAVANAI H. (2007), Study of the wettability properties of polypropylene nonwoven mats by low-pressure oxygen plasma treatment, Surf. Interface Anal. 39, 770–774. 23 CANAL C., GABORIAU F., MOLINA R., ERRA P. AND RICARD A. (2007), Role of the active species of plasmas involved in the modification of textile materials, Plasma Process. Polym. 4, 445–454. 24 NEGRI A. P., CORNELL H. J. AND RIVETT D. E. (1993), A model for the surface of keratin fibers, Text. Res. J. 63, 109–115. 25 KARAKAWA T., UMEHARA R., ICHIMURA H., NAKASE K. AND OHSHIMA K. (2002), Continuous shrink-resist process of wool top sliver with ozone 1 performance, Sen’i Gakkaishi. 58, 135–142. 26 WAKIDA T., LEE M., JEON J. H., TOKUYAMA T., KURIYAMA H. AND ISHIDA S. (2004), Ozone-gas treatment of wool and silk fabrics, Sen’i Gakkaishi. 60, 213–219. 27 WAKIDA T., LEE M., JEONG D. S., ISHIDA S. AND ITAZU T. (2003), Ammonia-gas treatment of cellulosic fabrics, Sen’i Gakkaishi. 59, 443–447. 28 WAKIDA T., TOKUYAMA T., DOI C., LEE M., JEONG D. S. AND ISHIDA S. (2004), Mechanical properties of polyester/cotton and polyester/rayon fabrics treated with ammonia-gas, Sen’i Gakkaishi. 60, 34–38. 29 LEE M., SUN LEE M. S., WAKIDA T., TOKUYAMA T., INOUE G., ISHIDA S., ITAZU T. AND MIYAJI Y. (2006), Chemical modification of nylon 6 and polyester fabrics by ozone-gas treatment, J. Appl. Polym. Sci. 100, 1344–1348. 30 ALISCH M., FEUERHACK A., BLOSFELD A., ANDREAUS J. AND ZIMMERMANN W. (2004), Biocatalytic modification of polyethylene terephthalate fibres by esterases from actinomycete isolates, Biocatal. Biotransform. 22(5), 347–352. 31 ANDREAUS J., NAU C. T., KISNER A., BUDAG N. AND BARCELLOS, I. O. (2004), Biocatalytic modification of polyamide fibers. In Proceedings of the 3rd International Conference on Textile Biotechnology, abstract 11. 32 MATAM´A T., SILVA C., O’NEILL A., CASAL M., SOARES C., GÜBITZ, G. M. AND CAVACO-PAULO, A. (2004), Improving synthetic fibers with enzymes. In Proceedings of the 3rd International Conference on Textile Biotechnology, abstract 5. 33 KHODDAMI A., MORSHED M. AND TAVANI, H. (2001), Effects of enzymatic hydrolysis on drawn polyester filament yarns. Iran, Polym. J. 10(6), 363–370. 34 YOON M. Y., KELLIS J. AND POULOUSE A. J. (2002), Enzymatic modification of polyester, AATCC Rev. 2, 33–36. 35 MIETTINEN-OINONEN A., SILVENNOINEN M., NOUSIAINEN P. AND BUCHERT J. (2003), Modification of synthetic fibres with laccase. In Proceedings of the 2nd International Symposium on Biotechnology in Textiles, abstract 13. 36 GENENCOR INTERNATIONAL (2001), Enzymatic modification of the surface of a polyester fiber or article, International Patent WO 0014629. 37 VERTOMMEN M. A. M. E, NIERSTRASZ V. A., VAN DER VEER M. AND WARMOESKERKEN

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M. M. C. G. (2005), Enzymatic surface modification of poly(ethylene terephthalate), J. Biotechnol. 120, 376–386. OUAJAI S. AND SHANKS R. A. (2005), Morphology and structure of hemp fibre after bioscouring, Macromol. Biosci. 5, 124–134. VÍLCHEZ S., JOVANCIC P., MANICH A. M., JULIA M. R. AND ERRA I. P. (2005), Chitosan application on wool before enzymatic treatment, J. Appl. Polym. Sci. 98, 1938–1946. IBRAHIM N. A., ALY A. A. AND GOUDA M. (2008), Enhancing the antibacterial properties of cotton fabric, J. Ind. Text. 37, 203–212. BUSCHMANN H.-J., KNITTEL D. AND SCHOLLMEYER E. (1992), German patent DE 40 35 378 A 1. K NITTEL D., B USCHMANN H.-J. AND S CHOLLMEYER , E. (1992), Neuartige AusrOstungseffekte für Natur- and Chemiefasern, Mal~geschneiderte Eigenschaften, Bekleidung Textil. 44(12), 34–40. NOSTRO P. L., FRATONI L AND BAGLIONI P. (2002), Modification of a cellulosic fabric with β-cyclodextrin for textile finishing applications, J. Inclusion Phenom. Macrocyclic Chem. 44, 423–427. LEE M. H., YOON K. J. AND KO S. W. (2001), Synthesis of a vinyl monomer containingcyclodextrin and grafting onto cotton fiber, J. Appl. Polym. Sci. 80, 438–446. GAWISH S. M., RAMADAN A. M., MOSLEH S. AND MORCELLET M. (2006), Synthesis and characterization of novel biocidal cyclodextrin inclusion complexes grafted onto polyamide-6 fabric by a redox method, J. Appl. Polym. Sci. 99, 2586–2593. DENTER U. AND SCHOLLMEYER E. (1996), Surface modification of synthetic and natural fibres by fixation of cyclodextrin derivatives, J. Inclusion Phenom. Mol. Recognit. Chem. 25, 197–202. VASANTHA M. D., EUNKYOUNG S. AND BEHNAM P. (2006), Surface modification of fibers and nonwovens with melt additives, in International Nonwovens Technical Conference, INTC 2006, pp 684–699. TORSTEN T., SCHOTER F. AND ECKHARD S. (2006), Functionalisation of textiles with nanotechnology, in Materials Research Society Symposium Proceedings, vol. 920, Smart Nanotextiles, pp 1–11. BECHERI A., DURR M., NOSTRO P. L, AND BAGLIONO P. (2008), Synthesis and characterization of zinc oxide nanoparticles: application to textiles as UV-absorbers, J. Nanopart. Res. 10, 679–689. LEROUX F., CAMPAGNE C., PERWUELZ A. AND GENGEMBRE L. (2008), Fluorocarbon nano-coating of polyester fabrics by atmospheric air plasma with aerosol, Appl. Surf. Sci. 254(13), 3902–3908. WEI Q., WANG Y., YANG Q. AND YU L. (2007), Functionalization of textile materials by pasma enhanced modification, J. Ind. Text. 36(4), 301–309.

2 Textile surface characterization methods Q . W E I , F . H U A N G and Y . C A I

Jiangnan University, China

Abstract: This chapter introduces the most commonly used techniques for the surface analyses of textile materials including microscopic methods, spectroscopic techniques and surface wetting measurement. The microscopic techniques used in the analysis of textile surfaces mainly include scanning electron microscopy (SEM), transmission electron microscopy (TEM), environmental scanning electron microscopy (ESEM) and scanning probe microscopy (SPM). The basic working principles and the applications of spectroscopic techniques for textile surface characterization are also explained. Wetting and contact angle measurement are also discussed in this chapter. The measurement techniques described are the sessile drop method, the use of barrel-shaped droplets on a single fiber and measurement of dynamic contact angles using the Wilhelmy technique. Key words: surface characterization, scanning electron microscopy (SEM), environmental scanning electron microscopy (ESEM), scanning probe microscopy (SPM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), contact angle.

2.1

Introduction

Technological innovations and commercial demands for high-quality products have been driving the textile industry towards sophisticated and diverse markets with versatile products for a wide spectrum of applications ranging from agricultural, filtration, civil engineering, medical and hygiene, packaging, protective clothing, sportswear, transport, defense, leisure and safety (Ajmeri and Ajmeri, 2002). For these expanding applications of textiles, their surfaces are specially engineered to give the product its desired properties. In fact, unsuitable surface properties may negate the otherwise advantageous bulk properties of a particular polymer. For example, poor surface hydrophilic properties will render a fiber unsuitable for producing underwear. Various techniques have been employed to improve the surface properties of textile materials, as discussed in other chapters in this book. The surface analysis methods for characterizing textile materials have been recognized as an essential process in the understanding and optimization of surface modification. Surface characterization of textiles has employed various techniques including optical, microscopic, spectroscopic, thermodynamic and mechanical analyses. 26

Textile surface characterization methods

27

The choice of which surface characterization method to use can be affected by a number of factors, such as material structures, the physical or chemical information required, sample preparation, or the analysis conditions, etc. The techniques used for examining the surfaces of textiles have been developing alongside the development of the textile industries. Based on their working principles, characterization methods for textiles can be divided into three major categories: (a) microscopic techniques used in the analysis of textile surfaces, primarily scanning electron microscopy (SEM), transmission electron microscopy (TEM), environmental scanning electron microscopy (ESEM) and scanning probe microscopy (SPM); (b) spectroscopic techniques, mainly Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) analysis; (c) surface wettability and other characterization methods. New developments in surface analysis technologies allow surface characterizations of the order of microns to be achieved. More recently, with the improvement of vacuum techniques and photoelectricity (electrons emitted from matter after the absorption of energy from electromagnetic radiation), more sensitive surface characterization at the angstrom and nanometer levels has become possible by using techniques such as SPM, XPS, etc. As it is believed that fiber surface behavior is determined by a surface layer of less than 10 nm thickness, it is clearly important to differentiate the properties of this thin layer from the bulk properties. In addition, techniques such as SEM, SPM and wettability analysis provide information about a range of surface properties of fibers important to their specific applications.

2.2

Surface characterization by advanced microscopies

2.2.1

Scanning electron microscopy

SEM is commonly used for examining the surface morphology and structures of textile surfaces (Giri, 2002). SEM makes use of a primary beam of electrons that interact with the specimen of interest, in a vacuum environment, resulting in the emission of secondary electrons, backscattered electrons, photons, X-rays, excitation of phonons and diffraction under specific conditions. The secondary electrons ejected from the specimen surface are collected and displayed to provide a high-resolution micrograph. The resolution and depth of field of the image are determined by a number of factors, such as beam current, beam energy, interaction volume and the final spot size, which can be adjusted as illustrated in Fig. 2.1. SEM scans a surface in the X–Y plane with a suitable detector and records the topography of the surface under observation with a resolution of the order of 1–2 nm and a magnification range from 102 to105. Information on structures,

28

Surface modification of textiles

Electron gun

Condenser lenses

Scan coils

Objective lens

Specimen stub

X-ray detector

Secondary electron detector

2.1 Schematic view of SEM.

compositions, phases and other properties can also be obtained with appropriate detectors. The utilization of SEM involves sample preparation, imaging and image analysis. SEM sample preparation requires fixation (hardening the specimen with a chemical or chemicals) followed by drying, attachment to a metallic stub as sample holder and then coating with a metal prior to imaging. The thin metallic coating, usually deposited by sputter coating, is typically 10–30 nm in thickness. Common conductive metals used include gold, platinum and gold–palladium alloy. It should be noted that the drying and metal coating processes used in the preparation of textiles might alter surface morphology, particularly fine surface features (Wei et al., 2004). Structural characterization of textiles by scanning electron microscopy Textile materials are usually designed and manufactured into various forms with different types of fibers to meet special requirements for a wide range of

Textile surface characterization methods

29

(a)

(b)

(c)

2.2 SEM observation of structured nanofibers: (a) aligned; (b) porous; (c) gradient.

applications. The structures of textiles are affected not only by fibers but also by the processing techniques involved. An understanding of the effects of fibers and processes on the properties of the finished materials is of importance in manufacturing textiles with the desired properties. The use of SEM techniques gives researchers and engineers insights into the fundamental understanding of the phenomena that have occurred. An example of the use of SEM in the structural characterization of textiles is presented in Fig. 2.2, which illustrates the observations of various structures of electrospun nanofibers (Wei et al., 2008). The aligned nanofibers were formed by controlling the electrospinning conditions and collecting set-up, as shown in Fig. 2.2(a). Aligned or oriented nanofibers have great potential in such applications as tissue engineering, high-strength nanocomposites, electronic and sensing applications. Porous nanofibers were also produced by using different solvents, controlling electrospinning conditions or environmental conditions. The porous nanofibers produced had much higher surface areas than conventional

30

Surface modification of textiles

(a)

(b)

(c)

(d)

2.3 Surface evolution of PAN nanofibers modified by sol-gel coating: (a) PAN nanofibers; (b) ZnO-coated PAN nanofibers; (c) pre-oxidized ZnO-coated PAN nanofibers; (d) carbonized ZnO-coated PAN nanofibers.

fibers, as shown in Fig. 2.2(b). The combination of porosity with flexibility in the porous nanofibers provides great potential for applications in many industries. Nanofibers containing a pore size gradient, as displayed in Fig. 2.2(c), have great potential for a wide range of applications in filtration and tissue engineering. SEM observations on the structural characteristics of the electrospun nanofibers demonstrate the applications of SEM in textile imaging. Surface characterization of textiles by scanning electron microscopy Textile surfaces have various properties, derived from the origin of the fibers and the way in which they are assembled. The surface characteristics of textiles are closely related to the phenomena of friction, wetting, dyeing, biocompatibility and other performance properties. Based on the understanding of surface properties, novel textiles and their applications may be created or engineered. SEM has been increasingly applied to examine textile surfaces at different levels. Figure 2.3 illustrates the use of SEM in the observation of the surface evolution of electrospun polyacrylonitrile (PAN) nanofibers modified by sol-gel coating of

Textile surface characterization methods (a)

31

(b)

2.4 Interfacial bonding between the sputter-coated layer and fiber: (a) ZnO–polyethylene terephthalate (PET) fiber; (b) ITO–PET fiber.

zinc oxide (Shao et al., 2008). The series of SEM images in Fig. 2.3 reveals the evolution of the surface nanostructures during the treatment process. The SEM image in Fig. 2.3(b) shows an obvious change in surface morphology of the PAN nanofibers after zinc oxide (ZnO) sol coating, compared with that in Fig 2.3(a). The coated PAN nanofibers have larger diameters than those of uncoated PAN nanofibers due to ZnO sol coating. In addition, the coarser nanofibers caused decreases in the pore sizes. The SEM image in Fig. 2.3(c) also indicates that the surface of pre-oxidized ZnO-coated PAN nanofibers became much rougher than those of ZnO-coated PAN nanofibers. Carbonization significantly altered the surface characteristics of the nanofibers, as displayed in Fig. 2.3(d). The ZnO coating formed larger clusters on the surface of the carbon nanofibers after carbonization. The SEM observation clearly reveals the evolution of the surface morphology of the nanofibers. Interface characterization of textiles by scanning electron microscopy Textile materials are always designed and manufactured to meet the demands of various applications. In these applications of textiles, interfaces are usually formed between two phases of either the same or different materials. The goal of textile interface studies is to understand the interfacial behaviors of textile materials and their resulting influence on material processes in order to facilitate the manufacture of technological textiles with optimized properties. The interfacial behaviors of textile materials are always associated with phenomena, such as friction, adhesion and adsorption. The SEM images in Fig. 2.4 represent the interfacial behaviors between textile fibers and deposited particles. As shown in Fig. 2.4(a), the SEM image indicates that on polyester fibers cracks formed in the coated ZnO layer. The ZnO coating was performed by magnetron sputter coating and cracks in the coating layer on the fiber surface reveal the poor bonding between the coating layer and the fiber.

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Surface modification of textiles

Better bonding between sputter-coated Indium tin oxide (ITO) and polyester fiber, as demonstrated in Fig. 2.4(b), was achieved by heating during the sputter coating. SEM provides a useful tool for observing the interfaces formed between textiles and other materials. Other characterization of textiles by scanning electron microscopy SEM has been consistently improved to meet the new demands for obtaining high resolution images of various structures of textile materials. The new generation of scanning electron microscopes provides compositional contrast observation using backscattered electrons. A clear contrast was obtained for blend polymers by employing the secondary electron image under a low accelerating voltage, but this also provided increased detail such as the lamellar structure (Goizueta et al., 1993). Through the use of backscattered electrons (BSEs) and a relatively low accelerating voltage on the scanning electron microscopy, the surface finish that is required to enable characterization has been found to be less demanding than that needed for secondary electrons. The identification of crystallographic phases in the scanning electron microscope has been limited by the lack of a simple way to obtain electron diffraction data of an unknown subject while examining the microstructures of a sample. With the development of charge-coupled device (CCD)-based detectors, electron backscattered diffraction (EBSD) patterns can be easily collected. Crystallographic phase analysis using EBSD permits high-magnification images, EBSDs and elemental information to be collected from bulk specimens (Goehner and Michael, 2004). SEM is a high-resolution imaging technique providing topographical and structural information in plan view or in cross-section. SEM is sometimes used in combination with an EDX analyzer. In the past, when functionally independent SEM and EDX techniques were used for analysis, equipment operation was complicated. The integration of SEM with EDX analysis has been achieved to enable simple operation, fast and clear SEM observations, and accurate EDX analysis (Yurugi et al., 2001).

2.2.2 Transmission electron microscopy (TEM) TEM is based on the fact that electrons can be ascribed a wavelength but at the same time interact with magnetic fields as a point charge. A schematic presentation of the transmission electron microscope is shown in Fig. 2.5. Using an electron gun, an electron beam is formed, which is accelerated by an electric field formed by a voltage difference of, typically, 200 kV. Electrons are scattered by the sample, situated in the object plane of the objective lens. Electrons scattered in the same direction are focused in the back focal plane, and, as a result, a diffraction pattern is formed there. Electrons coming from the same point on the object are focused in

Textile surface characterization methods

33

Electron gun

Condenser lens

Specimen grid

Objective lens

Projective lens

Phosphor screen

2.5 Schematic view of TEM.

the image plane. The image of a thin sample is formed by the electrons that pass the film without diffraction. Materials for TEM observations must be specifically prepared to thicknesses that allow electrons to be transmitted through the specimen. Samples for TEM in the form of films mounted on fine-meshed grids are required to be very thin. Moreover, the sample has to withstand the vacuum condition inside the transmission electron microscope. TEM sample preparation involves fixation, processing, embedding and sectioning (Radnoczi and Pecz, 2006). Embedding media used include methacrylates, polyester and acrylic resins, although epoxy resins are now commonly used. Specimens are typically sectioned using a microtome and need to be very thin since electrons with an accelerating voltage of 100 kV will not penetrate specimens more than 1000 nm thick. A modest transmission electron microscope can resolve in the sub-nanometer range with magnifications considerably higher than 2 × 105. It should however be noted that the embedding and sectioning processes used in the preparation of some polymeric materials may alter the materials themselves. The sample must also be able to withstand the electron beam.

34

Surface modification of textiles

(a)

(b)

1 µm

100 nm

2.6 TEM images: (a) PVAc nanofibers; (b) PA6–O-MMT nanofiber.

Surface and interface characterization of textiles by transmission electron microscopy The main use of the TEM technique is to examine fiber surface morphology, crosssections and interfacial phenomena in submicroscopic detail. Fine morphology with high resolution can be viewed after specimen preparation. The images in Fig. 2.6 illustrate the use of the TEM technique in observing textile materials. Fig. 2.6(a) shows the formation of an electrospun polyvinyl acetate (PVAc). A PVAc nanofiber and a barrel-shaped bead were observed, indicating the formation mechanism of jet stretching in the electropsinning. In another example, TEM observation also confirms the formation of composite nanofibers and the distribution of the organically modified montmorillonite (O-MMT) nanoparticles in a polyamide 6 (PA6) nanofiber matrix, as presented in Fig. 2.6(b). The TEM image indicates the nano-size O-MMT clay in the PA6 nanofibers electrospun from a solution containing 2 wt% O-MMT. It can be clearly observed that the O-MMT formed a dark line in the nanofiber. The TEM images also clearly reveal that the nanoclays were almost aligned along the nanofiber axis (Li et al., 2008). TEM has been increasingly used in the observation of nanostructured textiles. Crystallographic characterization of textiles by transmission electron microscopy In high-resolution transmission electron microscopy (HRTEM), it is also possible to produce an image from electrons deflected by a particular crystal plane. By either moving the aperture to the position of the deflected electrons, or tilting the electron beam so that the deflected electrons pass through the centered aperture, an image can be formed of only deflected electrons, known as a dark field (DF) image.

Textile surface characterization methods

35

In DF images, the direct beam is blocked by the aperture while one or more diffracted beams are allowed to pass the objective aperture. Information about planar defects, stacking faults or particle size can be obtained from DF images (Dobb et al., 1995). The TEM technique has been used to observe the texture, structure and chemistry of a boron nitride (BN) fiber (Chassagneux et al., 2002). The TEM analysis revealed that the general structure of the fiber consisted of two concentric parts: the finely nanocrystallized region near surface and the bulk with larger crystallites. Both hexagonal and rhombohedral BN forms have been clearly identified by the TEM observations. It was also found that all grains appeared to be randomly oriented with respect to the fiber axis, resulting in the modest mechanical properties of the fiber. Li et al. (2000, p. 8953) have synthesized a main-chain liquid crystalline polymer. The TEM observation revealed that crystallization taking place in the smectic phase could form both flat-elongated and double-twist helical single lamellar crystals. Justice et al. (2005, p. 4465) have also used TEM to examine the two-phase morphology in an acrylate-based system that developed during polymerization-induced phase separation (PIPS). It was found that interfaces developed from an acrylate-based recipe were more disordered than generally appreciated by using small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS), and the information gained from SAXS and USAXS was compared with data from SEM and TEM. Other characterization of textiles by transmission electron microscopy The capabilities of TEM have been increasingly extended by additional stages and detectors. A transmission electron microscope equipped with a cryo-stage is capable of maintaining the specimen at liquid nitrogen temperature. This allows the observation of frozen-hydrated samples. TEM can also be integrated with an EDX analyzer for detecting the elemental composition of the specimen.

2.2.3

Environmental scanning electron microscopy

In SEM, a focused beam of electrons is used to image the specimen and gain information as to its structure and composition. However, the entire column of the scanning electron microscope is normally under a high vacuum to minimize beamscattering effects. The high vacuum and the imaging process in SEM impose special requirements for specimen preparation. Coating specimens with a thin layer of a conductive material is often required for non-conductive specimens. One of the major disadvantages of SEM is that it is normally not possible to examine hydrated specimens or dynamic processes involving specimens in a wet state. Many technical applications of textiles in connection with liquids like grease, adhesives, water, oil, dyes and others cannot be examined. In attempts to overcome

36

Surface modification of textiles

Gun 10–7 Torr

10–5 Torr 10–4 Torr

10–1 Torr

10 Torr

Specimen chamber

2.7 Schematic view of ESEM.

these disadvantages, progress has been made in recent years in perfecting the environmental scanning electron microscope (Danilatos, 1980). The ESEM technique is able to image uncoated and hydrated samples by means of a differential pumping system, and a gaseous secondary electron detector (Danilatos, 1993). The differential pumping system shown in Fig. 2.7, enables the electron gun and upper parts of the column to be held at high vacuum (10–6–10–7 Torr), while the level of vacuum becomes progressively lower further down the column. Pressure limiting apertures (PLAs) allow the electron beam to pass through, but minimize the leakage of gases between zones pumped at different rates. Within the specimen chamber, pressures of up to 20 Torr can be maintained. ESEM can also be used to observe and record dynamic processes directly as they

Textile surface characterization methods (a)

37

(b)

2.8 Wetting behaviors of meltblown PP fiber: (a) by water; (b) by oil.

happen. Some accessories, such as a micro-injector, a heating stage and a mechanical testing stage can be added into an environmental scanning electron microscope to expand its observation capacities (Prack, 1993). Surface and interface characterization of textiles The ESEM technique is able to examine physically virtually any textile material without any special preparation or conductive coating. ESEM is also able to image not only non-conductive but also hydrated samples without any need for drying or coating. Therefore it is an important tool for interfacial studies of textile materials in a wet state. In ESEM, altering either pressure or temperature can change the relative humidity within the ESEM chamber. As relative humidity reaches 100% in the ESEM chamber, water is condensed on to the specimen surface. Wetting of fiber surfaces is fundamentally important in many different areas of textile science and engineering. Surface wetting properties govern the behavior of fibers in capillary sorption and transport fluids in fibrous media. The ESEM images in Fig. 2.8 indicate the different wetting behaviors of meltblown polypropylene (PP) fibers by water and oil. Water droplets form high contact angles on PP fiber surfaces, indicating the water-repellent property of PP fibers, as exhibited in Fig. 2.8(a). Bar-bell shapes were formed by oil droplets on the PP fibers, showing the oleophilic property of PP fibers, as presented in Figure 2.8(b). ESEM is also a useful tool for observing the biological properties of textiles. The example shown in Fig. 2.9 reveals the growth of bacterial cells on the poly-L-lactic acid (PLLA) fiber during the degradation of the fibers in a natural biodegradation process. Dynamic characterization of textiles ESEM is specifically suited to dynamic experimentation at the micron scale and below. ESEM technology allows dynamic experiments involving fluids, gases and

38

Surface modification of textiles

2.9 Biodegradation of PLLA fiber.

(a)

(b)

2.10 Water absorption of alginate fibers: (a) before water absorption; (b) after water absorption.

humidity, as well as the possibility of imaging samples that are undergoing compression and tension (Wei et al., 2002). The dynamic process of water absorption by alginate fibers is shown in Fig. 2.10. It was observed that the fiber absorbed water without forming any water droplets on the fiber surface when the humidity reached 100% in the ESEM chamber. The ESEM image also revealed the change in the fiber diameter. The water absorption led to an increase in fiber diameter, as shown in Fig. 2.10(b), compared to the image in Fig. 2.10(a).

Textile surface characterization methods

39

An environmental scanning electron microscope equipped with a heating stage can be used to observe the dynamic thermal processes in textiles. The heating stage is fixed in the ESEM chamber. The heating rate can be adjusted according to the test requirements. The installation of a micro-tensile stage in the environmental scanning electron microscope can also expand the observation capacities of the machine. The dynamic tensile behavior of various forms of textile materials can be examined under controlled conditions (Wei and Wang, 2003). The dynamic observations using ESEM will give more insight into the effect of the dynamic conditions on the surface evolution of textiles. Other characterization of textiles by environmental scanning electron microscopy The capabilities of the ESEM technique can also be significantly extended by additional stages and detectors. An environmental scanning electron microscope equipped with a cryo-stage is often used to observe biological samples. The adjustment of humidity, gases and temperature in the ESEM chamber makes it possible to scan textiles under various conditions – such as wetting, adsorption, hydration, degradation, oxidation and decomposition. The integration of ESEM with an EDX analyzer provides a new approach to examining the elemental composition of textile materials (Wei et al., 2004).

2.2.4

Scanning probe microscopy

Electron microscopy has long played a central role in the structural characterization of textiles and other materials, and indeed it is capable of giving a resolution of several angstroms. However, as the resolution increases, the field of view decreases and it becomes increasingly difficult to view fine structural details over a large area. Meanwhile, the requirements for a high vacuum and the need for a thin coating if an insulator is being observed, mean that certain types of materials are difficult or impossible to image by SEM. The invention of scanning tunneling microscopy (STM) has provided new tools in the microscopic analysis of materials. In STM, three-dimensional images of the surface topography of samples are obtained by monitoring the tunneling current flowing between an extremely sharp conductive probe and the sample surface. As the probe scans the surface, the magnitude of this current is inversely proportional to the probe–surface separation, with a change in the separation producing an order of magnitude change in the tunneling current. If the probe scans a raised area on the surface, the current increases. To compensate, a piezoscanner tube moves the probe tip, returning the tunneling current to its original value, as illustrated in Fig. 2.11(a). This technique also has advantages over other imaging techniques, such as SEM and TEM, in that no special sample preparation procedures are required.

40

Surface modification of textiles

(a)

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Piezoelectric tube with electrodes

Control voltages for piezotube

Amplifier Distance control and scanning unit

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2.11 Schematic view of : (a) STM; (b) AFM.

Atomic force microscopy (AFM) was invented a few years after the introduction of STM. AFM relies on a sharp tip that is scanned over a surface. This tip is part of a cantilever that can measure forces down to the lower piconewton range. During scanning, the forces between the surface and the tip cause the cantilever to bend in the vertical direction, and by measuring the power spectral density (PSD) of the deflection, it is possible to produce an image of the surface with atomic resolution. The forces, which can be attractive or repulsive, depend on the nature of the interaction between the tip and the surface being investigated, as shown in Fig. 2.11(b). AFM has three scanning modes: contact mode AFM, non-contact mode AFM and tapping mode AFM. In contact mode AFM, the tip on the cantilever scans the surface of a specimen in close contact with the sample. The deflection of the cantilever is detected for generating AFM images. In non-contact mode, the scanning tip hovers 5–15 nm above the specimen’s surface. The attractive forces acting between the tip and the specimen are measured for imaging. In tapping mode, the oscillating tip is moved toward the surface until it begins to tap the surface of a specimen. As the oscillating cantilever begins intermittently to contact the surface, the cantilever oscillation is reduced due to energy loss caused by the tip contacting the surface. The reduction in oscillation amplitude is used to examine the surface characteristics (Gibson et al., 1999).

Textile surface characterization methods

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2.12 Surface morphology of PET fiber: (a) before plasma treatment; (b) after plasma treatment.

Surface and interface analysis by atomic force microscopy AFM in different modes provides a new and powerful approach to the examination of the nanostructures of textile materials. The characterization of the nanostructures of textile materials is of importance in understanding their performance and structure–property relationships. An example of AFM scanning images is shown in Fig. 2.12, demonstrating the change in surface morphology caused by plasma treatment (Wei et al., 2007). The surface of the untreated PET fiber appeared to possess fibril structures. It can also be observed that the fibrils are oriented in the direction of the fiber axis, as indicated in Fig. 2.12(a). The etching effect of oxygen plasma treatment was

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Surface modification of textiles

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clearly observed, as shown in Fig. 2.12(b). The surface of the PET fiber was significantly roughened after the plasma treatment and the fibril structure was no longer visible. The surface roughness can also be calculated using the relevant software. AFM is also a useful tool in the analysis of interfacial phenomena. The AFM image in Fig. 2.13 illustrates the interface formed between a nano-coated layer and the mica surface.

Phase analysis by atomic force microscopy AFM in tapping mode provides a powerful phase-imaging function, which generates nanometer-scale information about surface composition, adhesion, friction, viscoelasticity and perhaps other properties. Phase imaging can be used to identify contaminants, to map different components in composite materials and to differentiate regions of high and low surface adhesion or hardness. The roughness of the solid surface is one of the main factors affecting the hydrophility of materials (Huang et al., 2006; Shibuichi et al., 1998). AFM images of electrospun PLLA–polyvinyl alcohol (PVA) nonwoven materials are shown in Fig. 2.14 (Liu et al., 2008), in which the surface structures of the two kinds of electrospun fibers are revealed. The surfaces of the nanofibers appear rough and the roughness is at the nanometre level, as shown in Fig. 2.14(a). The phase image of the same nanofibers looks quite different, as shown in Fig. 2.14(b), due to the different surface composition and viscoelasticity of the polymers.

Textile surface characterization methods

43

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2.14 PLLA–PVA composite nanofibers: (a) surface image; (b) phase image in AFM.

Force analysis by atomic force microscopy In addition to topographic and phase imaging, AFM can also be used to detect the amount of force felt by the cantilever as the probe tip is brought close to and even indented into a sample surface and then pulled away. This technique has been

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Surface modification of textiles

increasingly used to measure the attractive or repulsive forces between the probe tip and the sample surface, detecting local chemical and mechanical properties. Various forces can be detected using AFM techniques. Weisenhorn et al. (1989, p. 2651) compared the force between the atomic force microscope tip and a mica surface in air and water. It was found that measurement in air showed a large adhesive force due to capillary forces from the liquid contaminant layer on the surface. Once the whole cantilever probe was immersed in water, capillary forces were mostly eliminated and adhesion was substantially diminished. Butt (1991, p.1438) tried to measure the force between a silicon nitride cantilever and a mica surface under varying pH. It was found that at low pH there was a very strong attraction between the tip and sample. At high pH, the situation was reversed and long-range repulsion dominated. At an intermediate pH, the overall tip–sample forces could be minimized. Binding sensing molecules to the atomic force microscope tip for chemical force sensing has attracted great attention in recent years. Molecules bound to the atomic force microscope tip can be used as chemical sensors to detect forces between molecules on the tip and target molecules on the surface. Ducker (1992) has tried to use hydrophobic AFM colloidal probes to study the nature of the hydrophobic force. AFM and STM instruments have become the primary SPM instruments manufactured and used in various fields ranging from material research to biological analysis. However, an amazing number of instruments have been developed with different sensing techniques that permit detection of a wide range of surface properties in addition to topography. Several of these, lateral force microscopy (LFM), magnetic force microscopy (MFM) and chemical force microscopy (CFM) have great potential in textile science and research.

2.3

Surface characterization by advanced spectrometers

The surface properties of a material are significantly affected by the physical and chemical structure of the surface. The analysis of surface chemistry plays a very important role in the surface modification of textiles. Surface analysis by spectroscopic methods has been extensively applied in textile industries to obtain valuable information regarding the constituent elements and chemical structure near the surface of a sample. The principal modern techniques used in surface chemical analysis include Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Energy dispersive X-ray (EDX) analysis. These techniques provide analyses of the outermost atomic layers of a solid surface but each has its own dominance in different sectors of analysis.

2.3.1

Fourier-transform infrared spectroscopy

The infrared spectrum of a sample is collected by passing a beam of infrared light

Textile surface characterization methods (a)

45

(b) Pre-oxidized ZnO-coated PAN nanofibers

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2.15 FTIR spectra of: (a) PAN nanofibers and ZnO-coated PAN nanofibers; (b) pre-oxidized ZnO-coated PAN nanofibers and carbonized ZnO-coated PAN nanofibers.

through the sample. Examination of the transmitted light reveals how much energy is absorbed at each wavelength. This can be done by using a Fourier transform instrument to measure all wavelengths at once. From this, a transmittance or absorbance spectrum can be produced, showing at which infrared wavelengths the sample absorbs. Analysis of these absorption characteristics reveals details about the molecular structure of the sample. Infrared spectroscopy can be used to identify compounds or investigate sample compositions. It exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels. These resonant frequencies are determined by the shape of the molecular potential energy surfaces, by the masses of the atoms and by the associated vibronic coupling. Thus, the frequency of the vibrations can be associated with a particular bond type. Simple diatomic molecules have only one bond, which may stretch. More complex molecules have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups. Applications using FTIR spectroscopy in textiles include identification of unknown materials, contamination analysis, comparative analysis, analyzing the progress of a reaction and identification of surface chemical structures. The FTIR spectra of some fiber materials are presented in Fig. 2.15. They represent the FTIR spectra of PAN nanofibers, ZnO-coated PAN nanofibers, pre-oxidized ZnO-coated PAN nanofibers and carbonized ZnO-coated PAN nanofibers (Shao et al., 2008). The band at 2244 cm–1 is assigned to nitrile groups, while those at 2959 cm–1 and 1454 cm–1 are ascribed to the C–H stretching vibration, as shown Fig. 2.15(a). The absorption band situated at about 1735 cm–1 might be attributed to the C=O stretching vibration. The bands at 1572 cm–1,1573 cm–1,1408 cm–1 and 1405 cm–1 represent the asymmetric and symmetric C=O stretching, respectively. It has been observed that the bands at 2959 cm–1, 2244 cm–1 and 1735 cm–1 tend to decrease owing to chemical reaction of the pre-oxidized treatment. The FTIR

46

Surface modification of textiles

spectrum of carbon nanofibers is shown in Fig. 2.15(b).The band at 1591 cm–1 corresponds to C=C stretching vibrations, while the band at 453 cm–1 is correlated with ZnO. FTIR spectroscopy is a relatively easy technique and has been widely used in determining the chemical functionalities present in a sample. Monllor et al. (2007) used FTIR spectroscopy to determine the presence of microcapsules in textiles through the characteristic vibration properties of microcapsules displayed in FTIR spectra. A procedure based on FTIR spectroscopy was also proposed in this work to quantify the presence of microcapsules in modified fabrics by analysis of the intensity of characteristic waves. Kutanis et al. (2007) modified a PET fabric to endow it with conductive properties by means of surface polymerization. It was found through FTIR analysis that polyaniline was successfully attached to the fabric, and the amount of polyaniline deposited upon the PET fabric reached its maximum in 2 hours at 0 ºC and 20 ºC and in 1 hour at 40 ºC and 60 ºC. Owing to a significant increase in resolution, it is possible with FTIR spectroscopy to discriminate between contaminated and non-contaminated fibers in a textile. Abidi and Hequet (2007) investigated the use of the universal attenuated total reflectance-Fourier transform infrared (UATR-FTIR) technique to analyze cotton contaminations and to discriminate between sticky and non-sticky cotton. Abidi and Hequet (2005) also used UATR-FTIR spectroscopy to reveal that trehalose was the primary cause of defects in the formation of yarns during rotor spinning of moderately sticky cotton mixes. It was found from the results that trehalose was the primary concern in the formation of yarns from cotton with moderate honeydew contamination. Because of the complex chemical composition of modified textiles, the actual characterization of surface chemical structures using FTIR spectroscopy is sometimes very difficult. In order to simplify the complex spectra consisting of many overlapped peaks and enhance the spectral resolution by spreading peaks along a second dimension, two-dimensional correlation spectroscopy (Noda, 1993) can be / applied to analyze the FTIR spectra for textile characterization. Slusarczyk et al. (2007) tried to reveal the composition-induced structural changes in PA6–MMT composite fibers through two-dimensional correlation infrared spectroscopy. It was found that the content of MMT in the core of the fiber was higher than in the skin of the fiber and that the MMT might be pushed into the fiber core during the formation process of the PA6–MMT composite fibers.

2.3.2

X-ray photoelectron spectroscopy

Photoelectron spectroscopy utilizes photo-ionization and energy-dispersive analysis of the emitted photoelectrons to study the composition and electronic state of the surface region of a sample. Spectroscopic surface methods provide both qualitative and quantitative chemical information about the composition of a surface layer of a solid that is a few angstrom units to a few tens of angstrom units

Textile surface characterization methods 284.63 eV

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2.16 XPS spectrum of the PA6–O-MMT composite nanofiber.

in thickness. The presence of chemical bonding causes shifts in binding energy, that can be used to extract information of a chemical nature (such as atomic oxidation state) from the sample surface. For this reason, XPS is also known as electron spectroscopy for chemical analysis (ESCA). XPS has been widely used to determine the elemental composition of solid surfaces. Special sample preparation is generally not required for XPS, although surface contamination upon storage or during transport from the research laboratory to the XPS facilities may certainly have an adverse effect on the XPS results obtained. Figure 2.16 reveals the surface chemical structures of PA6–O-MMT composite nanofibers. It is clearly observed that the main elements of the surface species are C, N and O. The C1s binding energies are 284.6, 285.6 and 287.5 eV, which correspond to C–C/C–H, C–N (amine groups) and C=O (amide groups), respectively. However, the binding energies at 104.1 eV for Si2p (Si–O) and 73.8 eV for Al2p (Al–O) of silicate clay cannot be detected. These results confirm that the silicate clay is well dispersed within the composite nanofibers. Surface modification can alter the surface physical and chemical features of textile materials. Birdi (2003) found that plasma treatments led to the appearance of a shoulder at higher binding energies, which was taken as being an indication of

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Surface modification of textiles

the build-up of oxygenated carbon centers. Cai and Qiu (2003) used both He and O2 atmospheric plasma treatments to desize PVA on cotton. XPS analysis was applied to reveal the surface chemical changes including chain scission and formation of polar groups. It was found from the results that the O2 atmospheric plasma had a greater effect on PVA than the He plasma. Leroux et al. (2008) also used atmospheric plasma to modify fabrics. XPS was applied to characterize chemical surface modifications occurring as a result of the plasma treatments. Sawada et al. (2003) have fabricated a hydrophilic polyester fiber using photocatalytic reactions. The XPS analysis indicated that the photocatalytic reaction of TiO2 generated C–O and C=O groups on the surface of the polyester. However, owing to the heterogeneous chemical properties and a gauzy layer that is always less than 1 nm on the textile surface, XPS analyses are confronted with a challenge in modern characterizations. Therefore, the combination of surface chemistry with instrumental methods emerges as a powerful tool for the determination of functional groups on polymer surfaces (Hollander, 2004). Charret et al. (2002) attempted to coat a cotton fabric with a fluorinated resin. Any surface changes during the laundering of the cotton fabric were studied using a combination of low-frequency mechanical spectroscopy (LFMS), XPS and AFM. Mori and Inagaki (2006) used a combination of XPS and SEM to reveal the surface changes of wool fiber treated with plasma.

2.3.3

Energy dispersive X-ray analysis

EDX analysis is an analytical technique commonly used for the analysis of chemical compositions. The EDX technique analyzes X-rays emitted by a material when it is hit with electromagnetic radiation. In an EDX system, a high-energy beam is focused on the sample being studied. An atom within the sample contains unexcited electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole. The electrons and holes are attracted to opposite ends of the detector with the aid of a strong electric field. The size of the current pulse thus generated depends on the number of electron-hole pairs created. This in turn depends on the energy of the incoming X-ray, which is governed by the composition of the sample. Thus, an X-ray spectrum can be acquired giving information on the elemental composition of the material under examination. By moving the electron beam across the material an image of each element in the sample can be acquired. It is often necessary to identify the different elements associated with a textile. This is accomplished by using a ‘built-in’ EDX spectrometer. EDX analysis is often used in conjunction with electron microscopy during textile surface characterization. An EDX spectrum plot not only identifies the element corresponding to each of its peaks, but also the type of X-ray to which the peak corresponds. For example,

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a peak corresponding to the amount of energy possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is identified as a K-alpha peak. The peak corresponding to X-rays emitted by M-shell electrons going to the K-shell is identified as a K-beta peak. In textile characterization, EDX analysis has been used in quantitative elemental analysis (fixed-point, time-resolved, mapping) with a sensitivity down to a few atomic percent. The output of an EDX analysis is an EDX spectrum. The EDX spectrum is simply a plot of how frequently an X-ray is received for each energy level. An EDX spectrum normally displays peaks corresponding to the energy levels for which the most X-rays have been received. Each of these peaks is unique to an atom, and therefore corresponds to a single element. The higher a peak in a spectrum, the more concentrated the element is in the specimen. An example of EDX analysis in textiles is presented in Fig. 2.17. The EDX spectrum indicates the major elements of C, O, Ca and Na in alginate fibers, as shown in Fig. 2.17(a). The adsorption of metal ions was confirmed by the EDX spectrum shown in Fig. 2.17(b). The metal ions of Ni, Zn and Co are clearly shown in the EDX spectrum after ion exchange. This observation reveals the reduction of the Ca peak and the increase of the Na peak. As the most convenient technique of elemental identification, EDX analysis has been widely used in the characterization of the chemical compositions of textiles. Varesano et al. (2005) coated loose wool fibers with electrically conducting doped polypyrrole (PPy). EDX analysis was used to characterize the evenness and degradation, and the results indicated that fastness to organic solvents was excellent and a slight PPy decoating occurred during the manufacturing processes. Szymanowski et al. (2005) also used the EDX technique to investigate the elemental compositions of the titanium oxide coatings applied to cotton textiles. The analysis showed the composition of the textile coating to be close to that of titania with small contents of chlorine and carbon. The EDX technique has also

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Surface modification of textiles

been successfully applied in archaeology. Chen et al. (1998) used EDX analysis to examine the characteristics of the microstructures of archaeological and laboratory mineralized fibers. The EDX results revealed some similarities between the two types of fibers. Quantification of a particular element by analysis of the intensity of X-ray emitted is a technique commonly used in textile characterization. Shahidi et al. (2007) coated aluminum on cotton fabrics using low-temperature argon and oxygen plasmas. In their research, an EDX unit connected to a scanning electron microscope was used to determine the concentration of elements as atomic percentages present on the surface of the coated fabrics. By analyzing the intensity of each element, the aluminum content was easily revealed. El-Naggar et al. (2003) used an EDX unit to determine the atomic percentage of elements present on surface-coated fabrics.

2.4

Surface wetting and contact angles

Textiles have been increasingly used for applications involving wetting, such as wipes, sorbents, coalescers and filters. In these applications, surface wetting is of utmost importance. Wetting is the contact between a liquid and a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting can be described by the contact angle, at which the liquid–vapor interface meets the solid–liquid interface. It is defined geometrically as the angle formed by a liquid at the three-phase boundary where a liquid, gas and solid intersect, as described by Young-Dupré (Hartland, 2004) and as shown in Fig. 2.18. When the three-phase line is in motion, dynamic contact angles are formed, namely advancing and receding contact angles. The advancing angle is the contact angle when the three-phase line is moving forward, while the receding angle is the contact angle when the three-phase line is withdrawn over a pre-wetted surface. The difference between the advancing and receding contact angles is defined as the contact angle hysteresis (Tadmor, 2004).

Vapor

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2.18 Contact angle and interfacial tensions. γLV, interfacial energy between the liquid and the vapor phase; γSV, interfacial energy between the solid and the vapor phase; γSL, interfacial energy between the solid and the liquid.

Textile surface characterization methods

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2.19 Shape of droplet on polytetrafluoroethylene (PTFE) surface.

2.4.1

Sessile drop method

The sessile drop method is an optical contact angle technique used to estimate the wetting properties of a localized region on a solid surface. The angle between the baseline of the drop and the tangent at the drop boundary is measured. Figure 2.19 illustrates a lotus-like surface with gradient structures to enhance hydrophobicity and reduce hysteresis. As shown in Fig. 2.19, the static contact angle of water on the gradient roughness surface reached 152 ± 3º, indicating enhanced hydrophobicity, and the contact angle hysteresis drops to approximately 5º. This phenomenon can be explained by the mechanisms proposed by Cassie and Baxter (1944) and Wenzel (1936), which predicted that the wetting behavior of a surface can be enhanced by roughness or surface textures. The most common method of measuring the contact angle, the contact radius and the height of a sessile drop on a solid surface is to view the drop from its edge through an optical microscope. However, this method gives only local information in the view direction. Zhang and Yang (1983) developed a laser shadowgraphy method to investigate the evaporation of a sessile drop on a glass plate. Zhang and Chao (2002) improved the method and suggested a new optical arrangement to measure the dynamic contact angle and the instant evaporation rate of a sessile drop with much higher accuracy (less than 1% variation). With this method, any fluid motion in the evaporating drop can be visualized through shadowgraphy without using a tracer, which often affects the field under investigation. Wei et al. (2002) observed the wetting behavior of untreated and oxygen plasma-treated PP fibers using ESEM. Water droplets could be condensed on the fibers by adjusting

52

Surface modification of textiles Droplet X

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2.20 Barrel shape of droplet on fiber.

the humidity in the ESEM chamber. When the small droplets (in the micron size range) formed on fiber surfaces, the contact angles of water on the fiber surfaces were measured from the ESEM micrographs.

2.4.2

Barrel-shaped droplets on a single fiber

Liquid droplets on a fiber surface adopt shapes that conform to Laplace’s law (De Coninck et al., 2001) if the droplets are sufficiently small, so that gravity effects can be neglected. It is difficult to apply the tangent method described previously (Hartland, 2004) because of the meniscus curvature at the three-phase contact line, as shown in Fig. 2.20. The axisymmetrical barrel-shaped droplets on a single fiber can be analyzed using Carroll’s approach (Carroll, 1976). This approach to the calculation of contact angles of barrel-shaped droplets on cylindrical solids comprises an analytical expression relating droplet length L, maximum drop radius x2 and fiber radius x1, as shown in Fig. 2.20. – L – = aF(ϕ1,k) + nE(ϕ1,k) 2

(2.1)

– where L = L/x1, n = x2/x1, a = (ncos θ – 1)/(n – cos θ) and θ is the contact angle. k and ϕ1 are defined as k2 = 1 – (a2/n2) and sin2 ϕ1 = (n2 – 1)/(n2k2); F(ϕ1,k) and E(ϕ1,k) are elliptic integrals of the first and second kind, respectively. Oil contact angles on PP fibers were analyzed using this technique (Wei et al., 2003). It was found that the contact angles calculated were between 15º and 25º, indicating the affinity of PP fibers for oil. It was also found that with an increase in viscosity from light oil through to heavy oil, there was a small, but definite increase in contact angle.

Textile surface characterization methods

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53

Dynamic contact angle measurement using the Wilhelmy technique

The drop-shape techniques mentioned above apply only to the state in which the interfaces are stationary and the phase boundary line exists in equilibrium. When a phase boundary is in motion, such as in the case of a spreading droplet or advancing contact edge, different mechanics are involved. Dynamic wetting is of great importance in industrial processes. Dynamic contact angles are often used to describe the dynamic wetting behavior of a material. Dynamic contact angle measurement is usually performed using the Wilhelmy technique (Wilhelmy, 1863). When a solid is dipped into a liquid, the liquid will ascend (hydrophilic) or descend (hydrophobic) along the vertical side of the solid. The Wilhelmy method measures the pull force or the push force, i.e. the wetting force, to elucidate contact angles. The Wilhelmy technique has been widely applied to the analysis of dynamic contact angles. The use of this technique for the characterization of individual fibers is presented in Fig. 2.21. The effect of plasma treatment on the surface wettability of PP fibers was revealed by dynamic contact angle measurement. It can be seen from Fig. 2.21(a) that the untreated PP fibers had an advancing contact angle over 95º and receding contact angles of about 78º. The untreated fibers displayed hysteresis of about 17º, because of the surface roughness. The effect of plasma treatment on the contact angles is clearly revealed in Fig. 2.21(b). The advancing contact angle was reduced to about 83º and the receding contact angle was significantly lowered to about 52º after plasma treatment for 30 s. The hysteresis increased from about 17º for the untreated fibers to 31º for the treated fibers. This phenomenon is attributed to roughening of the fiber surface by plasma treatment (Huang et al., 2006).

2.5

Future trends

The surface properties of textiles can have an enormous impact on the performance of textiles in various applications. The use of appropriate surface analysis techniques to examine the surface properties of textiles – including structures, morphology, chemical features, wettability, etc. – is of paramount importance in the modification of textiles to meet the increasing demand for performanceenhanced materials. The surface modification of textiles has attracted a great deal of attention in recent years. For the surface modification of textiles, surface analysis methods are often required to understand the changes that have occurred in surface physical and chemical structures, to verify the effects of an intended surface modification technology as well as to correlate surface properties with the performance of materials. Microscopic and spectroscopic techniques each have their own advantages and disadvantages. With the development of new microscopic and

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spectroscopic techniques, new tools with improved performance will be tested and introduced for the surface characterization of textiles. However, a combination of surface characterization methods is often required to provide the comprehensive information necessary to enable a better understanding of the relationship between surface characteristics and material properties.

2.6

References

ABIDI N AND HEQUET E (2005), ‘Fourier transform infrared analysis of trehalulose and sticky cotton yarn defects using ZnSe-diamond universal attenuated total reflectance’, Textile Research Journal, 75, 645–652, doi: 10.1177/0040517505057527. ABIDI N AND HEQUET E (2007), ‘Fourier transform infrared analysis of cotton contamination’, Textile Research Journal, 77, 77–84, doi: 10.1177/0040517507074624. AJMERI J R AND AJMERI C J (2002), ‘Special textiles for industry applications’, Textile Magazine, 43(12), 70–72.

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BIRDI K S (2003), Scanning Probe Microscopes: Applications in Science and Technology, CRC Press, Boca Raton, Florida. BUTT H J (1991), ‘Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with an atomic force microscope’, Biophysical Journal, 60, 1438–1444, doi: 10.1016/S0006-3495(91)82180-4. CAI Z S AND QIU Y P (2003), ‘Effect of atmospheric plasma treatment on desizing of PVA on cotton’, Textile Research Journal, 73, 670–674, doi: 10.1177/004051750307300803. CARROLL B J (1976), ‘The accurate measurement of contact angle, phase contact areas, drop volume, and Laplace excess pressure in drop-on-fibre systems, Journal of Colloid Interface Science, 57, 488–492, doi:10.1016/0021-9797(76)90227-7. CASSIE A AND BAXTER S (1944), ‘Wettability of porous surfaces’, Transactions of the Faraday Society, 40, 546-551, doi: 10.1039/TF9444000546. CHARRET N, DAVID L, CAVAILLE J Y AND PERRIAT P (2002), ‘Washing durability of cotton coated with a fluorinated resin: an AFM, XPS, and low frequency mechanical spectroscopy study’, Textile Research Journal, 72, 832–843, doi: 10.1177/004051750207200913. CHASSAGNEUX F, EPICIER T, TOUTOIS P, MIELE P, VINCENT C AND VINCENT H (2002), ‘Texture, structure and chemistry of a boron nitride fibre studied by high resolution and analytical TEM’, Journal of the European Ceramic Society, 22, 2415–2425, doi:10.1016/ S0955-2219(02)00002-X. CHEN H L, JAKES K A AND FOREMAN D W (1998), ‘Preservation of archaeological textiles through fiber mineralization’, Journal of Archaeological Science, 25, 1015–1021, doi:10.1006/jasc.1997.0286. DANILATOS G D (1980), ‘An atmospheric scanning electron microscope (ASEM)’, Micron, 11, 335–336. DANILATOS G D (1993), ‘Introduction to the ESEM instrument’, Microscopy Research and Technique, 25, 354–361. DE CONINCK J, DE RUIJTER M AND VOUÉ M (2001), ‘Dynamics of wetting’, Current Opinion in Colloid & Interface Science, 6(1), 49–53, doi:10.1016/S1359-0294(00)00087-X. DOBB M G, GUO H AND JOHNSON D J (1995), ‘Image analysis of lattice imperfections in carbon fibers’, Carbon, 33, 1115–1120. DUCKER W A (1992), ‘Measurement of forces in liquids using a force microscope’, Langmuir, 8, 1831–1836. EL-NAGGAR A M, ZOHDY M H, MOHAMMED S S AND ALAM E A (2003), ‘Water resistance and surface morphology of synthetic fabrics covered by polysiloxane/acrylate followed by electron beam irradiation’, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 201, 595–603, doi: 10.1016/S0168583X(02)02069-4. GIBSON C T, WATSON G S, MAPLEDORAM L D, KONDO H AND MYHRA S (1999), ‘Characterisation of organic thin films by atomic force microscopy – application of force vs. distance analysis and other modes’, Applied Surface Science, 144–145, 618–622, doi: 10.1016/S0169-4332(98)00877-0. GIRI C C (2002), ‘Scanning electron microscope and its application in textiles’, Colourage, 49(3), 29–42. GOEHNER R P AND MICHAEL J R (2004), ‘Microdiffraction phase identification in the scanning electron microscope (SEM)’, Powder Diffraction, 19, 100–103, doi: 10.1154/ 1.1757450. GOIZUETA G, CHIBA T AND INOUE T (1993), ‘Phase morphology of polymer blends. II: SEM observation by secondary and backscattered electrons from microtomed and stained surface’, Polymer, 34, 253–256.

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HARTLAND S (2004), Surface and Interfacial Tension: Measurement, Theory, and Applications, Marcel Dekker, New York. HOLLANDER A (2004), ‘Labelling techniques for chemical analysis of polymer surfaces’, Surface and Interface Analysis, 36, 1023–1026, doi: 10.1002/sia.1828. HUANG F L, WEI Q F, WANG X Q AND XU W Z (2006), ‘Dynamic contact angles and morphology of PP fibres treated with plasma’, Polymer Testing, 25, 22–27, doi: 10.1016/ j.polymertesting.2005.09.017. JUSTICE R S, SCHAEFER D W, VAIA R A, TOMLIN D W AND BUNNING T J (2005), ‘Interface morphology and phase separation in polymer-dispersed liquid crystal composites’, Polymer, 46, 4465–4473, doi: 10.1016/j.polymer.2005.02.029. KUTANIS S, KARAKISLA M, AKBULUT U AND SACAK M (2007), ‘The conductive polyaniline/ poly(ethylene terephthalate) composite fabrics’, Composites: Part A, 38, 609–614, doi: 10.1016/j.compositesa.2006.02.008. LEROUX F, CAMPAGNE C, PERWUELZ A AND GENGEMBRE L (2008), ‘Fluorocarbon nanocoating of polyester fabrics by atmospheric air plasma with aerosol’. Applied Surface Science, 254, 3902–3908, doi: 10.1016/j.apsusc.2007.12.037. LI C Y, GE J J, BAI F, ZHANG J Z, CALHOUN B H, CHIEN L C, HARRIS F W, LOTZ B AND CHENG S Z D (2000), ‘Phase transformations in a chiral main-chain liquid crystalline polyester involving double-twist helical crystals’, Polymer, 41, 8953–8960, doi: 10.1016/ S0032-3861(00)00238-X. LI Q, WEI Q F, WU N, N, CAI Y B AND WEI D G (2008), ‘Structural characterization and dynamic water adsorption of electrospun polyamide6/montmorillonite nanofibers’, Journal of Applied Polymer Science, 107, 3535–3540, doi: 10.1002/app.27529. LIU Y, WEI Q F, WU N, SHAO D F, GAO W D (2008), ‘Preparation and structure of poly(lactic acid)/poly(vinyl alcohol) nanofiber’, China Synthetic Fiber Industry, 31(3), 5–7. MONLLOR P, BONET M A AND CASES F (2007), ‘Characterization of the behaviour of flavour microcapsules in cotton fabrics’, European Polymer Journal, 43, 2481–2490, doi: 10.1016/j.eurpolymj.2007.04.004. MORI M AND INAGAKI N (2006), ‘Relationship between anti-felting properties and physicochemical properties of wool fibers treated with Ar-plasma’, Textile Research Journal, 79(6), 687–694, doi: 10.1177/0040517506065590. NODA I (1993), ‘Generalized two-dimensional correlation method applicable to infrared, Raman and other types of spectroscopy’, Applied Spectroscopy, 47, 1329–1336, doi: 10.1366/0003702934067694. PRACK E R (1993), ‘An introduction to process visualisation capabilities and considerations in the environmental scanning electron microscope (ESEM)’, Microscopy Research and Technique, 25, 487–492, doi: 10.1002/ jemt. 1070250520. RADNOCZI G Z AND PECZ B (2006), ‘Transmission electron microscope specimen preparation for exploring the buried interfaces in plan view’, Journal of Microscopy, 224, 328–331, doi: 10.1111/j.1365-2818.2006.01707.x. SAWADA K, SUGIMOTO M, UEDA M AND PARK C H (2003), ‘Hydrophilic treatment of polyester surfaces using TiO2 photocatalytic reactions’, Textile Research Journal, 73, 819–822, doi: 10.1177/004051750307300912. SHAHIDI S, GHORANNEVISS M, MOAZZENCHI B, ANVARI A AND RASHIDI A (2007), ‘Aluminum coatings on cotton fabrics with low temperature plasma of argon and oxygen’, Surface Coating and Technology, 201, 5646–5650, doi: 10.1016/j.surfcoat.2006.07.105. SHAO D F, WEI Q F, ZHANG L W, CAI Y B AND JIANG S D (2008), ‘Surface functionalization of carbon nanofibers by sol-gel coating of zinc oxide’, Applied Surface Science, 254, 6543–6546.

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SHIBUICHI S, YAMAMOTO T, ONDA T AND TSUJII K (1998), ‘Super water- and oil-repellent surfaces resulting from fractal structure’, Journal of Colloid and Interface Science, 208, 287–294. / SLUSARCZYK C Z, BINIAS W, FABIA J AND BINIAS D (2007), ‘DSC and two-dimensional correlation infrared spectroscopy studies of PA6/montmorillonite composite fibres’, Fibres & Textiles in Eastern Europe, 15, 64–65. SZYMANOWSKI H, SOBCZYK A, GAZICKI-LIPMAN M, JAKUBOWSKI W AND KLIMEK L (2005), ‘Plasma enhanced CVD deposition of titanium oxide for biomedical applications’, Surface and Coatings Technology, 200, 1036–1040, doi: 10.1016/j.surfcoat.2005.01.092. TADMOR R (2004), ‘Line energy and the relation between advancing, receding and Young contact angles’, Langmuir, 20, 7659–7664. VARESANO A, ALUIGI A, TONIN C AND FRANCO FERRERO F (2006), ‘FT-IR study of dopantwool interactions during PPy deposition’, Fibers and Polymers, 7(2), 105–111. WEI Q F, MATHER R R, FOTHERINGHAM A F AND YANG R D (2002), ‘ESEM study of wetting of untreated and plasma treated polypropylene fibers’, Journal of Industrial Textiles, 31, 59–66. WEI Q F AND WANG X Q (2003), ‘Dynamic characterization of industrial textiles using an environmental scanning electron microscope’, Journal of Industrial Textiles, 33, 101– 110, doi: 10.1177/152808303038842. WEI Q F, MATHER R R, FOTHERINGHAM A F, YANG R AND BUCKMAN J (2003), ‘ESEM study of oil wetting behaviour of polypropylene fibres’, Oil & Gas Science and Technology – Revue de l’IFP, 58, 593–597, doi: 10.2516/ogst:2003041. WEI Q F, WANG X Q, MATHER R R AND FOTHERINGHAM A F (2004), ‘New approaches to characterization of textile materials using ESEM’, Fibres and Textiles in Eastern Europe, 12, 79–83. WEI Q F, LIU Y, HOU D Y AND HUANG F L (2007), ‘Dynamic wetting behavior of plasma treated PET fibers’, Journal of Materials Processing Technology, 194, 89–92. WEI Q F, HUANG F L, CAI Y B AND GAO W D (2008), ‘Structured polymer nanofibres and their potential applications’, Technical Textiles International, 5, 21–24. WENZEL R N (1936), ‘Resistance of solid surfaces to wetting by water’, Industrial & Engineering Chemistry, 28, 988–994. WEISENHORN A L, HANSMA P K, ALBRECHT T R AND QUATE C F (1989), ‘Forces in atomic force microscopy in air and water’, Applied Physics Letters, 54, 2651–2653, doi: 10.1063/ 1.101024. WILHELMY J (1863),‘Ueber die abhangigkeit der capillaritats-constanten des alkohols von substanz und gestalt des benetzten festen korpers’, Annalen der Physik, 119, 177–217. YURUGI T, SUKEHIRO I, YOSHINORI N AND SYKES K (2001), ‘The technology alliance for analytical instruments: SEM/EDX-integrated analysis system SEMEDX Series’, Readout, 22, 15–18. ZHANG N AND CHAO D F (2002), ‘A new laser shadowgraphy method for measurements of dynamic contact angle and simultaneous flow visualization in a sessile drop’, Optics and Laser Technology, 34, 243–248, doi: 10.1016/S0030-3992(02)00002-6. ZHANG N AND YANG W J (1983), ‘Visualization of evaporative convection in minute drops by laser shadowgraphy’, Review of Scientific Instruments, 54, 93–96. /

/

3 Textile surface functionalization by physical vapor deposition (PVD) Q . W E I , Y . X U and Y . W A N G

Jiangnan University, China

Abstract: Physical vapor deposition (PVD), especially sputtering technology, has been regarded as an environmentally friendly technique for the functionalization of textile materials. The basic principle of PVD is explained and the major applications of sputter coatings in the functionalization of textiles are also introduced in this chapter. Various functions can be achieved by using sputter coatings of metals, metal oxides and polymers. Composite coatings can also be obtained using co-sputtering. The functional textiles obtained include conductive, optical, magnetic and biocompatible materials for a wide range of applications. Some examples of depositing different functional films on different forms of textile substrates are presented, and the influences of major sputtering parameters on the properties of functionalized textiles are also discussed. This chapter also describes the microstructures of the interface between the coated film and the textile substrate. The bonding mechanism between the coated film and textile substrate is explained and confirmed by the peeling test. Key words: physical vapor deposition (PVD), magnetron sputtering, target, functions, coatings, interfaces.

3.1

Introduction

Textile materials in various forms have been increasingly used in many industries ranging from the automotive to the space industry. For a variety of applications it is desirable to produce such textile materials with specially designed surface properties. However, the naturally occurring surfaces of textiles are often not ideal for a particular application, such as providing anti-static, optical and other properties. In such cases various techniques have to be employed to modify the surface properties of textiles, based on changes to both the physical and chemical properties. The techniques used to modify textiles can generally be grouped into two major categories: chemical and physical. Chemical techniques usually involve careful control of the surface chemical environment or the addition of specific chemical species to modify the textile surfaces. Physical methods, however, generally tend to use non-chemical forces to control the etching or deposition of material. 58

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59

Physical forces – such as gas plasma, vaporization and irradiation – may be applied to modify textiles by adding or removing excess material from bulk assemblies. Of all these physical techniques, physical vapor deposition (PVD) stands out as a promising technique for the functionalization of textiles. PVD is basically a vaporization coating process involving an atom-by-atom or molecule-bymolecule transfer of material from the solid phase to the vapor phase and then deposition on to the surfaces of textiles. PVD techniques have been widely used to produce protective coatings, such as corrosion-resistant and wear-resistant coatings, as well as coatings for sensor applications. Multilayered and nanostructured coatings are always used to protect compressor or gas path components. Alternative coatings are often adopted to replace electro-plated hard chrome and cadmium, and some types of coatings are also made for high-speed machining tools. PVD techniques have been used extensively in metalworking (cutting tools, drills, saw blades, cold deformation and bending tools), automotive engine parts (gears, sprockets, piston rings, shafts, diesel injector pins, engine valves), plastics and rubber processing (molds and dies), injection and extrusion molding (plastics and aluminum; as mold inserts, rotating cores, slides, ejector pins), the aircraft and space industries (flap actuators), medical tools and implants and machine parts (Harsha, 2006; Helmersson et al., 2006). There are various processes considered as PVD technologies, such as evaporation, ion implantation and sputtering.

3.2

Working principles of physical vapor deposition

3.2.1

Vacuum evaporation

Vacuum evaporation is one of the most commonly used methods for deposition of functional films on to various substrates. The vacuum is used to allow vapor particles to deposit directly on to the substrate, where vapor particles condense back to a solid state, forming a functional coating. The vacuum evaporation process involves two basic stages: the evaporation of a functional material and the condensation on the substrate. In high-vacuum evaporation, electrical heating or electron beam heating is used to melt, gasify and evaporate the coating materials. The vapor of the coating material then travels to the surface of the substrate and gradually cools, a thin film layer of good quality is finally formed (Jankowski and Hayes, 2004). Vacuum is used to prevent the collision of the evaporated particles with the background gas or other unwanted particles. Evaporated functional materials deposit on to the surface of the substrate. Vacuum evaporation technology (VET) has been widely used to deposit functional films – such as those conferring resistance to wear, corrosion, high temperatures, oxidation and radiation; and those conferring enhanced conductivity, permeability and insulation properties – on various substrates. Evaporation techniques have also been used to produce functional plastics. A

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common example of this application is the metallization of polymer packaging films. The main purpose of the metallization of the packaging films is to isolate the product from the external environment by creating a barrier to the passage of light, oxygen or water vapor. Hong et al. (2001) used the evaporation technique to produce electromagnetic interference (EMI)-shielding textiles. Thermal vacuum evaporation of silver (Ag) was used to deposit a conductive layer on electrically insulating poly(ethylene terephthalate) (PET) woven fabric. The contribution of the absorbance or the reflectance to total EMI shielding efficiency was controlled through a coating of conductive polypyrrole (PPy) and the evaporated silver. VET uses a point-source to evaporate the coating materials, which restricts the use of metals and alloys with high boiling or melting points. Evaporation also has some other disadvantages, such as low bonding strength with the substrate.

3.2.2

Ion implantation

Ion implantation is a material surface modification process by which ions of a material are implanted into another solid material, causing a change in the surface physical and chemical properties of the materials. Ion implantation involves an ion source (where ions of the desired element can be produced), an accelerator (where the ions are electrostatically accelerated to a high energy) and a target (where the ions impinge on a target). The energy of the ions, as well as the ion species and the composition of the target, determine the functions acquired and the depth of penetration of the ions in the solid. Ion implantation has been widely used in the semiconductor and mechanical industries. The ion implantation of dopants, for example, is the most common application of ion implantation in the semiconductor industry. Nitrogen or other ions can also be implanted into steel materials. The structural and chemical changes caused by the implantation can prevent crack propagation and corrosion. The use of ion implantation in the surface modification of textiles has also been investigated in recent years. Wong et al. (2006) used ion implantation to modify electrospun poly(vinyl alcohol) (PVA) nonwovens. It was found that the ion implantation caused shrinking of the nanofibers, and the formation of two new functional chemical groups (N–C=O and C–N) in the PVA was observed. Öktem et al. (2008) modified PET membrane fabrics by Ti, W, Ti + N, Cr + N, and C + N implantations using a metal vapor vacuum arc (MEVVA) source at an accelerating voltage of 30 kV with doses ranging from 1 × 1015 to 5 × 1016 ions/cm2. It was found that the friction coefficient and wear loss values decreased significantly after implantation. The ions used in the ion implantation introduce both a chemical change in the target and a structural change. Ion implantation may cause damage to the crystal structure of the target, which is often not wanted.

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Target atoms Ar e

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e

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N

3.1 Schematic view of magnetron sputtering.

3.2.3

Sputtering

Sputtering, one of the most promising techniques in PVD technology, was first developed in the late 1970s. It has already been widely used to modify various materials in many industries. Sputtering is a process whereby atoms are knocked off from a solid target material due to bombardment of the target by energetic ions. These ejected atoms or molecules have a certain kinetic energy and orientation that cause them to condense on the substrate and then form a thin film (Kuniaki et al., 2000). The rate at which the coating material leaves the target depends on the number of bombarding ions hitting the target. It is therefore desirable to increase the plasma density in front of the sputtering source, to obtain a high deposition rate. Sputtering without any plasma confinement has the disadvantages of low deposition rates and low ionization efficiencies of the plasma. These limitations have led to the development of magnetron sputtering. Magnetron sputtering was developed to solve the electron problem by placing magnets parallel to the target surface, which can constrain the motion of secondary electrons ejected by the bombarding ions to the close vicinity of the target surface. The ion current is also increased by an order of magnitude over conventional diode sputtering systems, resulting in faster deposition rates at lower pressure (Ohring, 2002). A schematic diagram of magnetron sputter coating is shown in Fig. 3.1. Sputtering techniques can generally be grouped into categories depending on their functions. The techniques include direct current (DC) sputtering, radio frequency (RF) sputtering, reactive sputtering and magnetron sputtering. DC sputtering is usually used for metal targets. RF sputtering has to be used to avoid charge build-up when an insulating target is used. Magnetron sputtering utilizes

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strong electric and magnetic fields to trap electrons close to the surface of the target. Argon, an inert gas, is often used as the bombardment gas in a sputtering process. In reactive sputtering, the deposited film is formed by chemical reaction between the target material and a reactive gas that is introduced into the sputtering chamber. Sputtering has been extensively used in various industries to deposit thin films on different substrates. Terai et al. (2002) prepared trilayers of NbN–AlN–NbN with a critical current density (Jc) over 10 kA/cm2 using an RF sputtering method for the superconducting circuits based on Josephson junctions (two superconductors linked by a non-conducting barrier). Li et al. (2007) successfully deposited zinc oxide (ZnO) films on silicon, silicon dioxide and glass substrates by RF magnetron sputtering under different deposition conditions. It was found that the dark current of the ZnO metal–semiconductor–metal photodetector was reduced from 3.06 µA to 96.5 nA at 5 V after postdeposition annealing. The deposition of functional films on a flexible substrate by sputter coating has attracted a great deal of attention in recent years. Carneiro et al. (2007) used reactive magnetron sputtering to deposit pure and Fe-doped TiO2 thin films on to polycarbonate plates at different total pressures and iron-doping concentrations. The experimental results revealed that the TiO2 films deposited on the polycarbonate substrate only formed an amorphous structure. It was found that the highest photodegradation rates were obtained for films deposited on the polymer substrate under a lower total pressure of 0.4 Pa. Transparent indium-doped tin oxide (ITO) films were deposited on glass and polyethersulfone substrates by RF magnetron sputtering for plastic-based, flat-panel displays (Park et al., 2001). Highly conductive (20–25 Ω/square) and transparent (above 80%) ITO films deposited on polymer substrates were obtained under conditions of 0.2% oxygen partial pressure and vacuum annealing at a temperature of 180 ºC. In comparison with other deposition methods, a most important advantage of sputtering is that even the highest melting point materials are easily sputtered. Sputtered films typically have better adhesion on the substrate than evaporated films. The thickness of a sputtered film is much more easily controlled by fixing the operating parameters and simply adjusting the deposition time. Composite deposition can be easily accomplished by sputtering (Scholz et al., 2005; Wilmert and Hugo, 1999; Window, 1995). Sputter coating also provides the most promising technology for the surface functionalization of textile materials. Sputter coating offers a number of advantages over other technologies for textile materials: • an abundance of deposition materials exists, such as metals, metal oxides and polymers; • deposition takes place at low temperatures for polymer fibers; • deposited material adheres well to the fibrous substrates; • different deposition materials can also be combined.

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3.3

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Funtionalization of textiles by sputtering

Magnetron sputtering techniques are widely used to deposit different kinds of coatings, such as metallic coatings, oxide coatings, polymer coatings and composite coatings. These coatings are discussed in the following sub-sections.

3.3.1

Metallic coatings and their properties

Silver coatings Nearly all metallic materials can be deposited on textile substrates by sputtering. Silver, for example, has been widely used in the functionalization of textiles owing to its conductive, anti-bacterial and light shielding properties. Wei et al. (2004) investigated the surface morphology and wetting behavior of PET fibers functionalized by a sputter coating of silver. Atomic force microscopy (AFM) observations clearly revealed a significant difference in surface morphology before and after the silver sputter coating. It was also found that the morphology of the fibers changed with coating time and the growth of silver particles caused by the collision of the particles. The surface wetting behavior of the fiber was also altered by the introduction of the silver coatings, as revealed by environmental scanning electron microscopy (ESEM) imaging in wet mode. Wei et al. (2007b) further investigated the functionalization of spun-bonded polypropylene (PP) nonwovens by sputter coatings of copper and silver. It was found that the uncoated PP nonwoven had a transmittance of about 60% in the wavelength range from 300 nm to 600 nm, indicating good transmittance of visible light in the uncoated PP nonwoven. Sputter coatings with the metallic components of copper and silver, however, considerably reduced the transmittance in the materials, both in the ultraviolet (UV) and visible light ranges. The transmittance was further lowered as the coating thickness was increased. The research also revealed that the nonwoven substrates deposited with nanostructural copper had better UV and visible light absorption than those coated with silver at the same coating thickness. The better light shielding effect of the copper coating was attributed to the more compact and finer size of the copper nanoclusters formed on the PP fibers compared with the silver clusters on the fiber surface under the same sputtering conditions. The sputter coatings of copper and silver both significantly reduced the surface resistance. The surface resistance dropped from about 15.23 Ω cm to 0.87 Ω cm as the thickness of the copper coating increased from 20 nm to 100 nm. The surface resistance of the nonwoven material was reduced from 3.48 Ω cm to 0.24 Ω cm, as the silver coating thickness changed from 20 nm to 100 nm, indicating better surface conductivity than that of copper coating with the same thickness. The better surface conductivity of the nonwoven material coated with silver compared with those coated with copper, at the same coating thickness, was attributed to the better conductivity of silver itself. The anti-bacterial properties of textiles functionalized by silver sputter coating

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(a)

(b)

3.2 SEM images: (a) original PP fibers; (b) PP fiber sputtered with 3 nm silver film.

were also investigated (Wang et al., 2007a). The surface morphology of the nonwoven fibers was clearly altered by the silver sputter coating, as illustrated in Fig. 3.2. The surface of the original PP fibers looked smooth with some dust-like particles on it, as shown in Fig. 3.2(a). It was observed that the silver particles were scattered over the fiber surface after sputter coating, as indicated in Fig. 2.2(b). The anti-bacterial effect was investigated using the shake flash test and the test bacteria were Staphyllococcus aureus and Escherichia coli. It was found that all silvercoated PP nonwovens were very effective against both these test bacteria. The experimental results also indicated that the anti-bacterial performance significantly improved as the film thickness increased. It was believed that increasing the coating thickness clearly led to the release of a larger amount of silver ions, which contributed to the improved anti-bacterial performance. The investigation also revealed that the samples coated with silver films were much more effective against Staphylococcus aureus than against Escherichia coli. It was concluded that as the thickness of the silver film increased, the coverage of silver film improved and the release rate of silver ions increased, which finally led to the improvement of the samples’ anti-bacterial properties. The electromagnetic interference (EMI) shielding efficiency test also revealed the EMI shielding effect in PET nonwovens sputtered with silver. The results in Fig. 3.3 indicate the effect of coating thickness on EMI shielding efficiency. It is clearly shown that an increase in coating thickness led to a better EMI shielding effect. The functionalization of textile materials using a sputter coating of silver can significantly modify the surface properties of the materials. The development of modified materials with improved properties will open up new possibilities for applications of these materials. Copper coatings Textile materials consisting of polymer fibers provide an excellent platform for the

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65

EMI shielding efficiency (dB)

35 30

X ü

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3.3 Effect of coating thickness on EMI shielding efficiency.

integration of other functional structures to improve the performance of the materials. Sputter coating of copper has been carried out on textile materials (Wei et al., 2006b). The substrates used were nonwoven textile materials with three different fiber diameters: nanofibers, microfibers and normal fibers. The copper coating was performed by DC sputtering. The results of resistance measurements clearly indicated significant decreases in surface resistance for all samples after sputter coating. The surface resistance of all the samples before sputter coating was much higher than 106 Ω/cm, indicating the insulation properties of the fibrous materials. After coating for 10 min, the surface resistance of the normal fiber nonwoven textile dropped to about 250 Ω/cm and the microfiber and nanofiber webs showed surface resistances of less than 100 Ω/cm. It was found that fiber fineness had a certain effect on the surface resistance of the materials sputtered with copper. The substrate with finer fibers showed a lower resistance. This result was attributed to the greater number of fibers and fiber intersections in the web that consisted of finer fibers. The functionalization of textile materials using sputter coatings changes not only surface structures, but also surface properties. Wei et al. (2008a) investigated the effect of a sputter coating of copper on the surface roughness and dynamic contact angles of PET fibers. AFM observations revealed the formation of copper clusters deposited on the PET fiber surface. Growth of the sputtered particles was observed as coating thickness increased and this was attributed to the collision of the sputtered copper grains. It was also found that the surface roughness of the PET fiber changed with the coating thickness. The surface roughness of the untreated PET fiber was about 3.67 nm, as shown in Fig. 3.4(a). The surface roughness (measured by average distance between the surface and the meanline) increased to 18.35 nm as the coating reached 100 nm thick, as revealed in Fig. 3.4(b). The increase in surface roughness was caused by the growth of the copper clusters on the PET fiber surface.

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3.4 Surface roughness of: (a) the PET fiber; (b) the 100 nm copper coated fiber.

An increase in coating thickness led to a decrease in surface resistance. It was interesting to note the effect of the copper sputter coating on the dynamic contact angle of the PET fibers. Figure 3.5(a) shows the dynamic contact angles of the original PET fibers, with an average advancing contact angle of about 85º and receding contact angles of about 68º. There was obvious hysteresis between the advancing contact angle and the receding contact angle, on average about 17º. It was believed that the surface roughness of the PET fiber contributed to the contact angle hysteresis. It was observed from the dynamic contact angle measurement that the advancing contact angle was slightly lowered to about 70º after the sputter coating of copper was applied. The receding contact angles became much lower depending on the coating thickness. Thus, the receding contact angle was

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67

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3.5 Dynamic contact angles: (a) PET fiber; (b) PET fiber coated with 100 nm copper.

gradually reduced to about 32º (Fig. 3.5(b)) as the coating thickness reached 100 nm. It was calculated that the contact angle hysteresis increased from 17º to 38º. The change in the receding contact angle and the increase in hysteresis were attributed to the surface roughness. It is well known that fiber surface characteristics affect friction, wetting, conductivity and other performance properties. The evolution of the surface nanostructures of melt-blown PP fibers during the process of plasma treatment followed by copper sputter coating was investigated by Wei et al. (2007a). It was found that the modification altered the surface morphology and surface nanomechanical properties, as revealed by AFM and lateral force microscopy (LFM).

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Other metallic coatings In fact, various metallic films can be deposited on textile materials for a wide range of applications. Aluminum films have been deposited on PET spun-bonded nonwovens (Deng et al., 2007a). The change in surface morphology was revealed by AFM analysis. The formation of the aluminum coating was also confirmed by a full energy dispersive X-ray (EDX) analysis mounted on the environmental scanning electron microscope. It was found that the electrical resistance of aluminum-sputtered nonwoven material was significantly decreased. The substrate coated for 80 min showed much lower resistance (2.1 × 10 Ω/cm) than the substrate coated for 20 min (5.3 × 102 Ω/cm). Scholz et al. (2005) deposited precious metals of silver, copper, gold, platinum and platinum/rhodium on SiO2 fabrics by DC magnetron sputter coating. The layer thickness achieved was about 300 nm. The anti-bacterial properties of the coated fabrics were tested according to the appropriate standards (DIN 53 931 and AATCC Test Method 147-1998). The experimental results revealed that copper was most effective against bacteria and fungi. Silver was also effective against bacteria, but its effectiveness against fungi seemed to be limited. The other metals tested did not achieve any improved anti-bacterial properties.

3.3.2

Oxide coatings and their properties

Sputter coating, with its high deposition speed and excellent uniformity of nanofilms produced, has become one of the most commonly used techniques for the deposition of metal oxide films on various substrates including textiles. A wide range of metal oxide films can be deposited, such as TiO2, ZnO, indium-doped tin oxide and aluminum-doped ZnO. Titanium dioxide coating TiO2 thin films have been intensively studied in recent years since Fujishima and Honda (1972) reported the photocatalytic decomposition of water on TiO2. The advantageous photocatalytic properties of TiO2 are a result of the wide bandgap and long lifetime of the photogenerated holes and electrons. The deposition of TiO2 on to textiles has been found to produce interesting properties, owing to its unique dielectric and optical properties. Nanoscale TiO2 is an excellent photocatalyst and can be used in the catalytic oxidation of organic pollutants, sterilization, air cleaning and sewage treatment (Agugliaro et al., 1999). Nanoscale TiO 2 offers new opportunities for applications in chemical, biological, environmental protection and medical fields. Xu et al. (2007) used magnetron sputter coating to deposit nanoscale TiO2 functional films on the surfaces of cotton woven fabric and PET knitted fabric at room temperature. It was found that TiO2 coating resulted in some interesting

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properties in the PET substrates. The experimental results indicated that the fabrics with a TiO2 film deposit showed much better degradation of Methylene Blue under solar and UV radiation. It was also found that the cotton fabric showed better photocatalytic properties than the PET fabric. The existence of the hydrophilic groups in the TiO2 coated cotton fibers increased the amount of oxygen and hydrogen free radicals and oxygen negative ions, which played an important role in the TiO2 photocatalytic process. The experimental results also revealed that the TiO2 was easily activated by UV radiation at wavelengths below 400 nm. Yasuhiko et al. (1998) pointed out that the reaction rate was proportional to the irradiation intensity. TiO2 has also been deposited on polyamide 6 (PA6) nanofibers prepared by electrospinning (Wei et al., 2006a). The deposition was performed by RF sputter coating using a TiO2 target. The coating thickness of the deposition layer was measured using an Inficon XTM in situ film thickness monitor. The wetting behavior of the PA6 nanofibers was examined using ESEM. It was observed that water droplets on the surfaces of the TiO2 sputter-coated PA6 nanofibers looked like segments of spheres, prior to UV illumination, and so water contact angles were formed on the nanofiber surface. The effects of UV radiation were clearly discerned from the ESEM observation. After UV radiation for 2 h, the nanofibers coated for 10 min showed water films instead of water droplets on their surfaces, indicating the change in wettability of the nanofiber after UV illumination. TiO2 and PP nanocomposite fibers prepared by melt-compounding and sputter coating were compared by Wei et al. (2007c). It was found that incorporating the TiO2 by melt-compounding caused severe aggregation of the TiO2 nanoparticles at the PP fiber surface. In contrast, the coverage of the sputtercoated fiber surfaces was much more consistent, although aggregation of the TiO2 nanoparticles was still observed. The effect of UV irradiation on the hydrophilicity of the fiber surfaces was investigated using dynamic contact angle measurements and the Wilhelmy technique. The dynamic contact angle measurement revealed that the hydrophilicity of the PP fibers melt-compounded with TiO2 showed only a small increase, but a larger increase was noted for the PP fibers sputter coated with TiO2. It was believed that the surface coating covered the surface of the fibers more evenly, leading to a better photocatalytic activity under UV radiation. Sicha et al. (2008) also reported on the correlations between the process parameters of reactive pulsed DC magnetron sputtering and the physical properties and photocatalytic activity of TiO2 films. The experimental results indicated that TiO2 films with high photocatalytic activity could be created under lowtemperature reactive sputtering conditions. Textiles functionalized by TiO2 have great potential for applications involving the decomposition of various environmental pollutants in both gaseous and liquid phases. The development of TiO2-based photocatalysts anchored to fibrous materials with large surface areas, where pollutants could be eliminated or

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minimized, would be of great significance. Functional textiles loaded with nanoscale TiO2 not only avoid the disadvantages of filtration and suspension of fine photocatalyst particles that arise if TiO2 is used in isolation, but also provide a high photodecomposition efficiency (Uddin et al., 2007). Zinc oxide coating Thin films of ZnO have also attracted a great deal of attention in recent years owing to their excellent properties – such as high chemical stability, electrical conductivity and optical transparency in the visible range. It has been recognized as a promising alternative to transparent conducting ITO and TO because of its advantages in terms of low cost and non-toxicity (Ciobanu et al., 2006). ZnO has also been prepared by a number of processing techniques, such as ZnO composite fibers, sol-gel coating of ZnO thin film and chemical vapor deposition (CVD) of ZnO thin film. Sputter coating provides a new approach to the preparation of ZnO on textile substrates. Deng et al. (2007b) prepared a functional PET nonwoven material by magnetron sputter coating of ZnO. The coating was made using a ZnO target, and the effects of sputtering conditions on the surface morphology of the fibers were examined by AFM. The AFM images revealed an increase in the size of ZnO nanoclusters with increasing sputtering time. It was also found that an increase in the sputtering power led to an increase in the size of ZnO nanoclusters; sputtering pressure did not have any obvious effect on the size of the ZnO clusters. The optical properties of the ZnO-coated nonwoven material were also investigated and Fig. 3.6 shows the transmittance of UV and visible light through the material. The original nonwoven material shows a transmittance of over 70% in the wavelength range from 400 nm to 600 nm, indicating a good transmittance of visible light. The transmittance dropped gradually from 72% to about 5% in the wavelength range between 400 nm and 300 nm, indicating the UV shielding effect of the nonwoven material. The ZnO sputter coating significantly altered the optical properties of the nonwoven, as shown in Fig. 3.6. The transmittance of UV light in the range between 300 nm and 400 nm was considerably reduced when the thickness of the ZnO coating was 50 nm. An increase in coating thickness led to a further decrease in transmittance in both the UV and visible light range. The UV–visible light (UV–Vis) spectra also clearly indicated that the average transmittance of the ZnO-coated samples over the wavelength range between 400 nm and 600 nm exceeded 50%, which was very close to that of the original nonwoven, revealing the transparent property of the ZnO coatings in the visible light range. ZnO functional thin films can also be deposited on PET spun-bonded nonwoven material by DC reactive magnetron sputtering through the reaction of Zn with O2. The AFM images in Fig. 3.7 show the surface morphologies of the original sample and the sample with a ZnO coating. The images show a clear contrast in

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˜

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¤

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3.6 Effect of ZnO coating thickness (50, 100 and 200 nm) on the optical properties of a PET non-woven material.

surface morphology between the uncoated PET fibers and the ZnO-coated fibers. The surface of the original sample appeared quite smooth with the sporadic occurrence of grains, probably impurities such as dust adhering to the fibers. The surfaces of the ZnO-deposited samples, however, were covered with clearly recognizable nanoclusters. The UV–Vis spectra also revealed the transparent behavior of the ZnO films in the visible light range and the UV absorption effect of the ZnO coating. The results of an anti-static test are listed in Table 3.1. Semidecay time, which measures the time needed for the reduction of the static voltage on the fabric to reach half of the initial value, is used to represent the anti-static properties of a fabric. Therefore the shorter the semi-decay time, the greater the ability of the sample to neutralize the surface charge, indicating a better anti-static property. Compared with the original sample, the semi-decay time of the ZnOdeposited samples was considerably shortened, indicating that the anti-static properties were greatly improved. Table 3.1 also reveals that the semi-decay time clearly decreased as the deposition thickness increased, i.e. anti-static properties are enhanced by the increase in coating thickness.

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Surface modification of textiles (a) 900.00 nm 800.00 nm 700.00 nm 600.00 nm 500.00 nm 400.00 nm 300.00 nm 200.00 nm 100.00 nm 0.00 nm

3000.00

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3.7 AFM image of: (a) PET fiber; (b) fiber coated with ZnO using reactive sputtering.

Indium-doped tin oxide coating Doping has been widely accepted as a technique for the modification of metal oxides in order to improve their performance. Doping can generate systems of a host metal oxide with a low concentration of dopant incorporated into the host

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Table 3.1 Effect of coating thickness on anti-static properties

Sample Original sample 100 nm deposition 150 nm deposition

Peak voltage (V)

Static voltage (V)

Semi-decay time (s)

2600 4080 3740

2637 4090 3780

99.0 9.55 2.72

structure. The doped substance, therefore, often exhibits different properties from the host material, and presents unique applications that are only possible with the doped system. ITO is a typical example of a doped metal oxide. ITO films have been increasingly used in a variety of applications, such as organic light-emitting diodes (OLED), liquid crystal displays (LCD), electromagnetic shielding, anti-UV applications and so on. In these applications, the performance of the device is strongly affected by the properties of the ITO. ITO films have been deposited on glasses and polymer substrates. Wang et al. (2007b) deposited a functional coating of ITO on textile substrates. They used PET spunbonded nonwovens as substrates and the coating was performed by magnetron sputtering using an ITO target. ITO films with thickness of between 50 nm and 224 nm were prepared on PET spun-bonded nonwovens. All were deposited under the same power (150 W), pressure (0.5 Pa), temperature (150 ºC) and Ar flow rate (20 sccm). AFM observations revealed the ITO particles deposited on the PET substrate. ITO sputter coatings of different thickness significantly altered the optical properties of the PET nonwoven material. The uncoated sample shows a transmittance of about 70% in the wavelength range from 400 nm to 600 nm, indicating good transmittance of visible light. The transmittance drops gradually from 70% to less than 5% in the wavelength range between 400 nm and 300 nm, indicating the UV shielding effect of the PET material. The transmittance of the PET sample decreased as the coating thickness increased from 56 nm to 224 nm, as illustrated in Fig. 3.8. The transmittance of the material dropped from about 70% to 60% in the visible light range between 400 nm and 600 nm, when the ITO coating thickness was 56 nm. The transmittance decreased gradually from 60% to less than 5% in the wavelength range between 400 nm and 300 nm, when the coating thickness was about 56 nm. The transmittance was further reduced as the ITO coating thickness was increased to 224 nm. The transmittance reduced to about 50% at wavelengths between 400 nm and 600 nm, and the UV absorption was also improved as the coating thickness was increased to 224 nm. The effect of the coating thickness on the surface resistivity is shown in Fig. 3.9 (Wang et al., 2007b). The surface resistivity of the PET nonwoven coated with an ITO film of 20 nm was about 136.2 kΩ cm, a significant drop from the resistivity of over 106 kΩ cm for the PET nonwoven. As the film thickness increased to

74

Surface modification of textiles 80 PET 56 nm

Transmittance (%)

70 60 50 112 nm

40

224 nm

30 20 10 0 300

350

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3.8 Optical properties of an ITO-coated PET non-woven material.

160 140




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3.9 Resistivity of an ITO-coated PET non-woven material.

100 nm, the resistivity rapidly decreased to 1.24 kΩ cm. The surface resistivity then showed a much slower decrease to about 1 kΩ cm as the coating thickness was further increased from 100 nm to 200 nm. This phenomenon is also supported by the findings of Kim et al. (2006). Hao et al. (2008) have also successfully deposited ITO thin films on PET substrates and investigated the structural, electrical and optical properties of these films. The difference between this technique and that used by Wang et al. (2007b)

Textile surface functionalization by physical vapor deposition

75

is that DC reactive magnetron sputtering was used here and the substrate was treated by plasma cleaning before deposition. It was found that as the film thickness increased, the average crystal grain size increased, but the transmittance, the resistivity and the sheet resistance decreased. The potential of transparent conducting oxides attracted a lot of attention in recent years owing to their great promise in many industries. Transparent conducting oxides also have great potential in the textile industry for technical applications. Aluminum-doped zinc oxide (AZO) coating AZO thin films deposited on various substrates have also attracted a great deal of attention in recent years due to their attractive properties combined with low cost. Deng et al. (2008) deposited AZO by RF magnetron sputtering on PET spunbonded nonwoven materials. The nonwoven samples were ultrasonically cleaned in ethanol and distilled water before the sputter coatings were applied. The sputter coating of the functional layer was carried out on a magnetron sputter coating system using an AZO target, which was mounted on the cathode. The PET spunbonded nonwoven samples were fixed on the sample holder with a distance of 60 mm between target and substrate. AFM images obtained showed the significant change in surface morphology of the PET fibers. The UV–Vis spectra revealed the effect of sputtering time on the optical properties of the AZO-coated nonwovens. It was found that the favorable characteristic of transparency of the AZO coatings was achieved. It was also observed that the coating thickness had an obvious effect on the transmittance of the samples in the wavelength range from 300 nm to 400 nm, indicating the ability of the AZO coatings to absorb ultraviolet UV light. The UV absorption is attributed to the characteristics of direct transition-type semiconductors and the optical bandgap of the AZO film. The experimental results indicated that the room temperature resistivity of the sputtered AZO nonwoven gradually decreased as the sputtering time increased. The electrical resistivity dropped to 5.2 × 103 Ω cm from 50 × 103 Ω cm, as the deposition time increased from 30 min to 90 min. The fall in resistivity was attributed to the improvement in the compactness and homogeneity of the nanofilms deposited on nonwovens. AZO thin films have also been deposited on glass and polycarbonate (PC) substrates by RF magnetron sputtering (Lee et al., 2007). High-quality films with resistivities as low as 9.7 × 10–4 Ω cm and transmittance values over 90% have been obtained by suitably controlling the RF power. Other metal oxide coatings Various other metal oxides have been investigated as functional coatings for different substrates. Cai et al. (2008) coated PA6–organically modified

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2.5 µm

3.10 SEM image of PA6–O-MMT composite nanofibers modified by Fe2O3 magnetron sputter coating.

100

Weight (wt%)

80

60

40

20

PA6–OMMT–Fe2O3 PA6–Fe2O3 PA6

0 100

200

300

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500

600

700

Temperature (°C)

3.11 Differential scanning calorimetry (DSC) analysis of PA6–O-MMT composite nanofibers.

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77

montmorillonite (O-MMT) composite nanofibers using an Fe2O3 magnetron sputter technique. SEM images revealed that the ellipsoidal-like Fe2O3 nanoparticles were deposited on the surfaces of nanofibers, as illustrated in Fig. 3.10. Thermogravimetric analysis (TGA) indicated the improved thermal stability of the composite nanofiber, owing to a synergistic effect between O-MMT and Fe2O3, as indicated in Fig. 3.11. It was suggested that Fe3+ cations facilitated decomposition of hydroperoxides through a reversible oxidative–reductive catalytic process between Fe3+ and Fe2+. Meanwhile, the increased charred residue contributed to the improved thermal stability of the coated PA6–O-MMT composite nanofibers.

3.3.3

Polymer coatings and their properties

Sputtering, one of the most promising techniques in PVD technology, has great potential for depositing different polymer coatings. Plasma polymerization processes have increasingly been used for the deposition of thin films of plasma polymers which have great potential for a variety of applications ranging from surface modifications to protective films in electronics. The deposition of polymeric materials by RF sputtering has become a promising process for many industries. Gaseous organic fragments used as precursors for plasma polymerization processes are knocked off the conventional polymer target (Biederman and Slavínská, 2000). Fluorocarbon plasma polymers are often used because of their good dielectric films, low friction and protective and optical properties. Deposition of polytetrafluoroethylene (PTFE) by means of RF sputtering has been performed on glass substrates. The films were deposited on aluminum-pre-coated glass substrates in argon and nitrogen, and in a self-sputtering mode. The average temperature of the target was found to be below the melting point of PTFE. Wettability measurements, using the contact angles of water droplets, revealed that static contact angles approaching 105º and 100º were found for fluorocarbon plasma polymer films sputtered in argon and nitrogen, respectively (Biederman et al., 2001). PTFE coatings have also been investigated with textile substrates. Huang et al. (2007) described the surface functionalization of woven silk fabric by magnetron sputter coating of PTFE. The PTFE sputter coating was applied to improve the hydrophobic properties of the silk fabric. The effects of PTFE sputter coating on surface morphology and surface chemical properties were characterized using AFM and ATR-FTIR (attenuated total reflection-Fourier transform infrared) spectroscopy. The wettability of the fabric was characterized by measuring the surface contact angle using a dynamic sessile analysis (DSA) technique. Contact angle hysteresis was also investigated in this work using dynamic contact angle measurements. As shown in Fig. 3.12, the advancing contact angle changed from 110º to 129º, while receding contact angle increased from 84º to 123º as the argon pressure was constantly increased from 5 Pa to 50 Pa. Figure 3.12 reveals an obvious decrease in contact hysteresis as the working pressure increased. It was

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Contact angle (degrees)

140 130 120 110



















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3.12 Dynamic contact angles of silk fabric coated with gradient PTFE.

believed that the surface roughness contributed to the change in contact angle hysteresis.

3.3.4

Composite coatings and their properties

Technical demands have driven the sputtering industry into a new stage of development. Nanocomposite coatings provided by the application of sputtering technology have also opened up new possibilities in the surface modification and functionalization of various materials for a variety of applications. In recent years, much attention has been paid to the deposition of nanocomposite coatings for improving mechanical properties. The structure, composition and hardness of reactively sputtered W–B–N thin films, for example, were investigated by X-ray diffraction (XRD), electron probe microanalysis (EPMA) and Vickers ultramicroindentation. The chemical composition of the films was changed from W80B20 to W42B9N49 by varying the partial pressure of N2. All the as-deposited coatings were amorphous, except the W60B17N23 film, which showed crystalline peaks, indexed as body-centered cubic (bcc) α-W, overlapping the amorphous feature. This nanocomposite structure led to a maximum as-deposited Vickers hardness of 36 GPa (Louro et al., 2005). Metal and plasma polymer composite films prepared by sputter coating have been investigated. Composite films of SiOx–fluorocarbon plasma polymers were prepared by RF sputtering from two balanced magnetrons equipped with PTFE and silica (SiO2) targets. The SiOx–fluorocarbon plasma polymer composite films showed different wettabilities (static contact angle of water ranges from 68º to 40º) and Brinell hardness values (ranges from 720 to 3200 N/mm2) when the volume fraction ratio (filling factor) of SiO2 changed from 0.01 to 0.7 (Pihosh et al., 2006a).

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Pihosh et al. (2006b) have also investigated composite SiOx–hydrocarbon plasma polymer films prepared by RF sputtering of silica and hydrocarbon polymer (polyethylene (PE) or PP) targets placed on two balanced magnetrons. It was found that the nanocomposite films demonstrated a wide range of wettability and hardness, depending on the preparation conditions. In addition, many other kinds of composite coatings have been studied. For example, Chernenko et al. (2008) prepared sub-micrometer Ni–Mn–Ga films on MgO single-crystalline wafers by RF magnetron sputtering. Besseghini et al. (2008) deposited Ni51.4Mn28.3Ga20.3 thin films on alumina ceramics, and Pierson and Horwat (2007) successfully grew Ag–Cu–O films on glass substrates by reactive magnetron sputtering.

3.4

Interfacial bonding

Sputter coating can deposit a very thin layer of a coating on to a wide range of textile substrates. The sputtered atoms have a high energy and when they impinge on any surface, they form a surface coating. The adhesion between the coated layer and the substrate plays a very important role in various applications of the sputtercoated materials.

3.4.1

Mechanism of adhesion

It has been found that particle adhesion between coating and substrate or among particles mainly involves van der Waals forces, diffusion, mechanical interaction and electrostatic attraction. Van Der Waals forces are physical forces and can be divided into static electric force, inductive force and dispersion force. Diffuse adhesion is generated in a process, during which atoms are diffused into each other by heating, bombarding ions and other forces. Mechanical interaction refers to a macroscopic effect between the sputtered particles and the substrate surface. There are a number of factors that may influence the adhesion between the sputtered substance and the substrate. The first important factor is the deposition technique used. Films deposited on the same substrates but using different methods will generate different adhesion forces. For example, the adhesion force of a sputtered film is usually much higher than that of an evaporated film. In addition, the deposition conditions, such as power used, deposition speed and the angle of deposited particles arriving at the substrate surface all have some influence. The second important factor is the deposition temperature, which may affect the adhesion force by changing the interface conformation. When film is deposited at low temperature, the adhesion force is mostly determined by mechanical adhesion and van der Waals forces, and it will decrease as the space between atoms increases. Higher temperature can not only cause atom transfer on the surface of the substrate and their diffusion into each other, but can also accelerate gas and

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3.13 SEM image of the interface of ITO-coated fibers.

impurities to pull off the substrate (impurities include surface dust, oil, water, etc.). All these factors will enhance the film’s adhesion force. However, too high a temperature will increase thermal stress and then decrease adhesion force. Meanwhile, high temperature will also enlarge grains of film and affect other characteristics of the films. A third factor influencing adhesion is surface conditions. If grease, oil and other impurities remain on the surface of a substrate, the adhesion force will decrease. Therefore, cleaned substrates always have better adhesion with films than uncleaned substrates. Yet another factor is the interface formed between film and substrate. Different substrates have different bonding types and wetting properties so that the adhesion between film and substrate is also variable. In addition, interface stress, internal stress and growth stress can also influence the adhesion force to a greater or lesser extent.

3.4.2

Microstructures of the interfaces

The microstructures of the interfaces between the coated layer and the substrate play an important role in the bonding of a coating layer. Figure 3.13 illustrates the microstructures of surface and interface; the ITO-coated fibers show a smooth surface, but the cracks at the edge of the fiber indicate the existence of the interface formed by the ITO sputter coating. The interfacial bonding structures between the coated clusters and fibers were also observed by AFM (Wei et al., 2008b). The images in Fig. 3.14 indicate the interfacial structures of PP fibers after the peel-off test. The interface without

Textile surface functionalization by physical vapor deposition

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(a)

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3.14 Interfacial structures observed using AFM: (a) coated fiber; (b) plasma pre-treatment; (c) fiber heated during sputtering.

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Surface modification of textiles

plasma pre-treatment or heating during the sputter coating showed separation of the sputtered copper layer from the fiber surface after the peel-off test, indicating poor adhesion between the sputtered copper nanoclusters and the PP fibers (Fig. 3.14(a)). Some unpeeled nanoclusters remained on the fiber surface. The interface between the sputtered copper layer and the PP fiber pre-treated with plasma appeared much rougher after the peel-off test, compared with that of the untreated fibers. There were clearly more copper clusters remaining on the fiber surface after the peel-off test, as shown in Fig. 3.14(b), revealing the improved adhesion between the coated layer and the PP fibers. It was believed that the rough surface and functional groups formed on the fiber surface by plasma pretreatment contributed to the better adhesion of the coated material. The interface formed when heating was used during the sputter coating process showed different structures (Fig. 3.14(c)). The sputtered copper nanoclusters appeared embedded in the PP fiber matrix, indicating the enhanced adhesion of the coated layer to the PP fibers. It was believed that the use of heating during the sputter coating of copper facilitated the diffusion of the copper nanoclusters into the PP fibers, leading to better adhesion of the coated layer. Jayaram et al. (1994) investigated interfacial structures using high-resolution electron microscopy and observed the structures of sputter-deposited MoS2 coatings under both conventional and ultra-high vacuum (UHV) conditions. As deposited, the films had a mixture of short-range ordered, basal-plane- and edgeplane-oriented grains near the film–substrate interface. It was observed that substantial long-range ordering of the basal islands followed by grain growth occurred during thermal annealing in UHV and in oxygen. It was found that gold nucleation and growth on both thermally annealed and as-deposited films followed the Volmer–Weber mode, i.e. three-dimensional islands that appeared highly textured. Gold demonstrated higher stability to electron beam fluxes on MoS2 substrates compared with results on carbon and SiO substrates, suggesting higher bonding strengths on the MoS2 substrate.

3.4.3

The adhesion force

The bonding between a sputter-coated layer and the fibers is usually tested by the abrasion test and the peel-off test. Peel-off test Peel-off tests have been used in attempts to analyze the adhesion force of the sputter-coated films on textile substrates (Wei et al., 2008b). The test was performed on a Zwick universal materials testing machine to examine the interfacial adhesion of the coated layer. The test speed was set at 200 mm/min. The initial opening distance between the tape and the textiles was 10 mm. The tape used was 3M-600 test bonding tape. The test samples were cut into 7 cm × 2.5 cm

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Table 3.2 Adhesion force Sputtering conditions Sample number

Material

Thickness (nm)

Temperature (ºC)

Bonding force (N)

1-1 1-2 1-3 2-1 2-2 2-3 2-4 3-1 3-2

PET PET PET PET PET PET PET Tape Cotton, plain

50 100 200 100 100 100 100 100 100

Room temperature Room temperature Room temperature 50 100 150 200 Room temperature Room temperature

3.65 3.98 4.52 3.95 4.25 4.42 4.06 7.25 4.95

rectangles for the peel-off test. The samples were pressed with a load of 400 g for 12 h before the peel-off test. All the tests were performed at 20 ± 2 ºC and 65 ± 2% relative humidity (RH). Each test was carried out three times and average values were recorded. Three groups of samples, all deposited with ITO films, were selected. The first group contained PET spun-bonded nonwoven samples coated with ITO in different thicknesses (50 nm, 100 nm and 200 nm). The second group contained PET samples sputter coated with ITO at different temperatures (50 ºC, 100 ºC, 150 ºC and 200 ºC). The last group used different substrates (plastic tape, cotton fabric and PET spun-bonded nonwovens). The results in Table 3.2 indicate that the adhesion force increased from 3.65 N to 4.52 N as the thickness increased from 50 nm to 200 nm at room temperature. The coating thickness contributed to the adhesion force owing to the compactness of the thick coating. It was also found that the adhesion force was enhanced as the substrate temperature increased (Table 3.2). The adhesion force reached a maximum value at 150 ºC and then showed a slight decrease as the temperature was further increased to 200 ºC. This phenomenon should be attributed to the change in the crystallinity of the ITO films and interaction between the ITO coating and the PET fiber surface. The increase of adhesion force with the increase in the substrate temperature could also be explained by the fact that the grain size increased significantly with the increase in deposition temperature, thus reducing grain boundary scattering and increasing conductivity (Nisha et al., 2005). The structure of the substrate also affected the bonding between the coated layer and the substrate, as shown in Table 3.2. The plastic tape showed the largest adhesion force among the samples tested. This is because the tape was a film with a flat surface for the ITO coating, but the cotton fabric and the PET nonwoven were porous materials. It was observed that the PET spun-bonded nonwovens deposited with ITO films had the lowest adhesion force. The lower bonding action of the ITO

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Surface modification of textiles

coating with the PET fibers, compared with the bonding to cotton fibers, may be the reason for the results obtained. Li et al. (2004) demonstrated the benefits of plasma treatment for realizing extremely strong adhesion between gold thin films and a polysiloxane-based polymer. Plasma treatment was used to modify the polymer surface and it was found that the polymer surface was roughened greatly by the plasma treatment. The pull-off strength of the gold film from the polymer was enhanced from 1.2 N/mm2 to 8.5 N/mm2 after the plasma treatment. Further improvement in the adhesion was achieved by sputter-coating two adhesion layers, Al2O3–Al, on the polymer before coating the gold film and annealing the films at 150 ºC for 2 h. The pull-off strength of the gold film from such a polymer–Al2O3–Al–Au system was over 35 N/mm2. It was believed that the extremely strong bonding between the gold and the other layers of the system was obtained mainly by the formation of Au–Al alloys at the interface between the aluminum and gold layers, based on an analysis using chemical etch and EDX spectroscopy.

Abrasion test The abrasion test has also been used to investigate the adhesion between the sputtered layer and the substrate (Mitterer et al., 1991). Wei et al. (2008b) used the abrasion test to investigate the adhesion between the sputtered layer and textile substrates as follows. Abrasion testing of copper coatings on PP nonwoven materials was performed on a Zweigle G552 abrasion tester. The abrasion load was 30 g. The size of the sample was 20 cm × 1.0 cm. All the tests were performed at 20 ± 2 ºC and 65 ± 2% RH. Each test was carried out three times and the average values were reported. The samples used are listed in Table 3.3 and the results of the abrasion test are given in Table 3.4. The results clearly indicate the effects of plasma pre-treatment and heating on the abrasion resistance of the copper-coated PP nonwoven material. The original PP nonwoven had the lowest abrasion resistance of the samples tested. The abrasion resistance of the PP nonwoven sputter coated with copper was significantly improved, as shown in Table 3.4. The higher abrasion resistance was ascribed to the formation of a metal layer on the fiber surfaces. It was also observed from Table 3.4 that plasma pre-treatment clearly increased the abrasion resistance of the material, but the pre-treatment for 90 s caused a decrease in the abrasion resistance. The roughening effect of the plasma pre-treatment facilitated the bonding between the sputtered nanoclusters and fibers. However, the longer exposure period of 90 s caused decomposition of the fibers, as shown by AFM; the abrasion resistance of the material was also therefore weakened to some extent. Table 3.4 also indicates the effect of heating on abrasion resistance during the sputtering process. It was found that heating also enhanced the bonding between the sputtered nanoclusters and the fiber, but raising the temperature may cause damage to the fibers themselves.

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Table 3.3 Sample details Sample

Plasma pre-treatment

Heating during sputtering

Copper coating

1 2 3 4 5 6 7 8

No No 30 s 60 s 90 s No No 30 s

No Room temperature Room temperature Room temperature Room temperature 40 ºC 80 ºC 40 ºC

No 100 nm 100 nm 100 nm 100 nm 100 nm 100 nm 100 nm

Table 3.4 Abrasion resistance Sample

Abrasion resistance (number of wear cycles)

1 2 3 4 5 6 7 8

975 1846 2545 2878 2476 2643 2436 2748

Other methods Coating adhesion is of importance in the application of functional textiles. The adhesion of coatings to substrates can be determined qualitatively or quantitatively depending on the test method used. Various other methods have also been developed to study adhesion phenomena, such as scratching (Thouless, 1998) and indenting (Marshall and Evans, 1984) the surface of the coating. Scratch testing continuously measures the force and displacement of a microprobe, generally a diamond tip, to generate an interfacial crack and spilling. Nanoindentation can be used either to induce spontaneous buckling of a film or to create indentation blisters, depending on the residual stress of the film. Lin et al. (1998) used indentation adhesion to investigate the adhesion of diamond films deposited on cemented WC + (3–5)% Co substrates with Ti–Si interlayers. The results showed that Ti–Si could be a good interlayer to improve film adhesion and inhibit diffusion of Co to the substrate surface on diamond nucleation. This was due to the formation of strong TiC and SiC bonding to enhance film adhesion.

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3.5

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Future trends

Sputtering has been used extensively in many industries, such as in semiconductors, microelectronics, packaging and other areas. Because of the low substrate temperatures used, sputtering is an ideal method for the deposition of functional layers on polymer substrates. The use of sputter coating to deposit highquality transparent conductive oxides such as ITO at room temperature on flexible organic substrates, for example, has attracted a great deal of attention in recent years. This is fuelled mainly by emerging technologies such as flexible flat panel displays (FFPDs) and organic light-emitting diodes (OLEDs). Conventional deposition techniques often require elevated substrate temperatures, rendering them unsuitable for deposition on heat-sensitive polymer substrates (Anguita et al., 2007). Sputter coatings have also been increasingly used in the deposition of thin films on flexible substrates for sensing applications. Akiyama et al. (2006) have investigated the highly sensitive piezoelectric response of c-axis-oriented aluminum nitride (AlN) thin films prepared on PET membranes using a RF magnetron sputtering method at temperatures close to room temperature. The sensor consisting of AlN and PET films was flexible like the PET membranes, and the electrical charge was linearly proportional to the stress within a wide range from 0 MPa to 8.5 MPa. Kim et al. (2006) have also reported high-performance organic thin film transistors (OTFTs) fabricated on flexible substrates such as plastic films and photo papers, using various deposition techniques including sputter coatings. Sputter coatings also provide new approaches to the functionalization of textiles, using metallic, oxide, polymer and composite coatings to achieve various performance properties. The most important advantage of sputter deposition is that even the highest melting point materials can be easily sputtered on polymer substrates at low temperature. Nanocomposite coatings can also be easily obtained by the co-sputtering of various materials. The sputtering process for the functionalization of textiles is usually performed only on the side facing the target, owing to the directional deposition of the technique. This prevents the deposition of the functional coating on to the inner layers of thick textile materials.

3.6

References

AGUGLIARO V, COLLUCCIA S, LODDO V, MARCHESE L, MARTRA G, PALMISANO L AND SCHIAVELLO M (1999), ‘Photocatalytic oxidation of gaseous toluene on anatase TiO2 catalyst: mechanistic aspects and FT-IR investigation’, Applied Catalysis B: Environmental, 201, 15–27, doi: 10.1016/S0167-577X(98)00150-5. AKIYAMA M, MOROFUJI Y, KAMOHARA T, NISHIKUBO K, TSUBAI M, FUKUDA O AND UENO N (2006), ‘Flexible piezoelectric pressure sensors using oriented aluminum nitride thin films prepared on polyethylene terephthalate films’, Journal of Applied Physics, 100, 114318, doi: 10.1063/1.2401312.

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ANGUITA J V, THWAITES M, HOLTON B, HOCKLEY P, RAND S AND HAUGHTON S (2007), ‘Room temperature growth of indium-tin oxide on organic flexible polymer substrates using a new reactive-sputter deposition technology’, Plasma Processes and Polymers, 4, 48–52, doi: 10.1002/ppap.200600047. BESSEGHINI S, CAVALLIN T, CHERNENKO V, VILLA E, LVOV V AND OHTSUKA M (2008), ‘Variation of atomic spacing and thermomechanical properties in Ni–Mn–Ga/alumina film composites’, Acta Materialia, 56, 1797–1801, doi: 10.1016/j.actamat.2007.12.025. BIEDERMAN H AND SLAVÍNSKÁ D (2000), ‘Plasma polymer films and their future prospects’, Surface and Coatings Technology, 125, 371–376, doi: 10.1016/S0257-8972(99)00578-2. BIEDERMAN H, ZEUNER M, ZALMAN J, BÍLKOVÁ P, SLAVÍNSKÁ D, STELMASUK V AND BOLDYREVA A (2001), ‘Rf magnetron sputtering of polytetrafluoroethylene under various conditions’, Thin Solid Films, 392, 208–213, doi: 10.1016/S0040-6090(01)01029-X. CAI Y B, HUANG F L, WEI Q F, WU E C AND GAO W D (2008), ‘Surface functionalization, morphology and thermal properties of polyamide6/O-MMT composite nanofibers by Fe2O3 sputter coating’, Applied Surface Science, 254, 5501–5505, doi: 10.1016/ j.apsusc.2008.02.185. CARNEIRO J O, TEIXEIRA V, PORTINHA A, MAGALHÃES A, COUTINHO P, TAVARES C J AND NEWTON R (2007), ‘Iron-doped photocatalytic TiO2 sputtered coatings on plastics for self-cleaning applications’, Materials Science and Engineering: B, 138, 144–150, doi: 10.1016/j.mseb.2005.08.130. CHERNENKO V A, BESSEGHINI S, HAGLER M, MULLNER P, OHTSUKA M AND STORTIERO F (2008), ‘Properties of sputter-deposited Ni–Mn–Ga thin films’, Materials Science and Engineering A, 481–482, 271–274, doi: 10.1016/j.msea.2006.12.206. CIOBANU G, CARJA G, APOSTOLESCU G AND TARABOANTA I (2006), ‘Structural, electrical and optical properties of thin ZnO films prepared by chemical precipitation’, Superlattices and Microstructures, 39(1–4), 328–333, doi: 10.1016/j.spmi.2005.08.058. DENG B Y, WEI Q F, GAO W D AND YAN X (2007a), ‘Surface functionalization of nonwovens by aluminum sputter coating’, Fibres & Textiles in Eastern Europe, 15(4), 90–92. DENG B Y, YAN X, WEI Q F AND GAO W D (2007b), ‘AFM characterization of nonwoven material functionalized by ZnO sputter coating’, Materials Characterization, 58, 854– 858, doi: 10.1016/j.matchar.2006.08.002. DENG B Y, WEI Q F AND GAO W D (2008), ‘Physical properties of Al-doped ZnO films deposited on nonwoven substrates by RF magnetron sputtering’, Journal of Coatings Technology Research, 5(3), 393–397, doi: 10.1007/s11998-008-9087-7. FUJISHIMA A AND HONDA K (1972), ‘Electrochemical photolysis of water at a semiconductor electrode’, Nature, 238, 37–38, doi: 10.1038/238037a0. HAO L, DIAO X G, XU H Z, GU B X AND WANG T M (2008), ‘Thickness dependence of structural, electrical and optical properties of indium tin oxide (ITO) films deposited on PET substrates’, Applied Surface Science, 254, 3504–3508, doi: 10.1016/ j.apsusc.2007.11.063. HARSHA K S S (2006), Principle of Vapor Deposition of Thin Films, First Edition, Elsevier Ltd, Oxford, UK. HELMERSSON U, LATTEMAN M, BOHLMARK J, EHIASARIAN A P AND GUDMUNDSSON J T (2006), ‘Ionized physical vapor deposition (IPVD): a review of technology and applications’, Thin Solid Films, 513, 1–24, doi: 10.1016/j.tsf.2006.03.033. HONG Y K, LEE C Y, JEONG C K, SIM J H, KIM K, JOO J, KIM M S, LEE J Y, JEONG S H AND BYUN S W (2001), ‘Electromagnetic interference shielding characteristics of fabric complexes coated with conductive polypyrrole and thermally evaporated Ag’, Current Applied Physics, 1, 439–442, doi: 10.1016/S1567-1739(01)00054-2.

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HUANG F L, WEI Q F, LIU Y, GAO W D AND HUANG Y B (2007), ‘Surface functionalization of silk fabric by PTFE sputter coating’, Journal of Materials Science, 42, 8025–8028, doi: 10.1007/s10853-007-1580-3. JANKOWSKI A AND HAYES J (2004), ‘The evaporative deposition of aluminum coatings and shapes with grain size control’, Thin Solid Films, 447–448, 568–574, doi: 10.1016/ j.tsf.2003.07.018. JAYARAM G, DORAISWAMY N, MARKS L D AND HILTON M R (1994), ‘Ultrahigh vacuum high resolution transmission electron microscopy of sputter-deposited MoS2 thin films’, Surface and Coatings Technology, 68–69, 439–445, doi: 10.1016/02578972(94)90199-6. KIM D H, PARK M R, LEE H J AND LEE G H (2006), ‘Thickness dependence of electrical properties of ITO film deposited on a plastic substrate by RF magnetron sputtering’, Applied Surface Science, 253, 409–411, doi: 10.1016/j.apsusc.2005.12.097. KUNIAKI T, MAO K AND HIROAKI U (2000), ‘Preparation of ITO electrode on the organic layer by sputtering’, Electronics and Communications in Japan, 83, 23–30. LEE J Y, LEE D J, LIM D G AND YANG K J (2007), ‘Structural, electrical and optical properties of ZnO:Al films deposited on flexible organic substrates for solar cell applications’, Thin Solid Films, 515, 6094–6098, doi: 10.1016/j.tsf.2006.12.099. LI M Y, CHOKSHI N, DELEON R L, TOMPA G AND ANDERSON W A (2007), ‘Radio frequency sputtered zinc oxide thin films with application to metal–semiconductor–metal photodetectors’, Thin Solid Films, 515, 7357–7363, doi: 10.1016/j.tsf.2007.03.026. LI W T, CHARTERS R B, LUTHER-DAVIES B AND MAR L (2004), ‘Significant improvement of adhesion between gold thin films and a polymer’, Applied Surface Science, 233, 227– 233, doi: 10.1016/j.apsusc.2004.03.220. LIN C, KUO C T AND CHANG R M (1998), ‘Improvement in adhesion of diamond films on cemented WC substrate with Ti–Si interlayers’, Diamond and Related Materials, 7, 1628–1632,doi: 10.1016/S0925-9635(98)00204-0. LOURO C, LAMNI R AND LÉVY F (2005), ‘W–B–N sputter-deposited thin films for mechanical application’, Surface and Coatings Technology, 200, 753–759, doi: 10.1016/ j.surfcoat.2005.02.132. MARSHALL D B AND EVANS A G (1984),‘Measurement of adherence of residually stressed thin films by indentation I. Mechanics of interface delamination’, Journal of Applied Physics, 56, 2632–2638, doi: 10.1063/1.333794. MITTERER C, ÜBLEIS A AND EBNER R (1991), ‘Sputter deposition of wear-resistant coatings within the system Zr-B-N’, Materials Science and Engineering A, 140, 670–675. NISHA M, ANUSHA S, ANTONY A, MANOJ R AND JAYARAJ M K (2005), ‘Effect of substrate temperature on the growth of ITO thin films’, Applied Surface Science, 252, 1430–1435, doi: 10.1016/j.apsusc.2005.02.115. OHRING M (2002), Materials Science of Thin Films, Academic Press, Oxford, UK. ÖKTEM T, TARAKÇLO GLU I, ÖZDO GAN E, ÖZTARHAN A, NAMLLGÖZ ES, KARAASLAN A AND TEK Z (2008), ‘Modification of friction and wear properties of PET membrane fabrics by MEVVA ion implantation’, Materials Chemistry and Physics, 108, 208–213, doi: 10.1016/j.matchemphys.2007.09.032. PARK S K, HAN J I, KIM W K AND KWAK M G (2001), ‘Deposition of indium–tin-oxide films on polymer substrates for application in plastic-based flat panel displays’, Thin Solid Films, 397,49–55, doi: 10.1016/S0040-6090(01)01489-4. PIERSON J F AND HORWAT D (2007), ‘Influence of the current applied to the silver target on the structure and the properties of Ag–Cu–O films deposited by reactive cosputtering’, Applied Surface Science, 253, 7522–7526, doi: 10.1016/j.apsusc.2007.03.054. V

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PIHOSH Y, BIEDERMAN H, SLAVINSKA D, KOUSAL J, CHOUKOUROV A, TRCHOVA M, MACKOVA A AND BOLDYRYEVA A (2006a), ‘Composite SiOx/fluorocarbon plasma polymer films prepared by r.f. magnetron sputtering of SiO2 and PTFE’, Vacuum, 81, 38– 44, doi: 10.1016/j.vacuum.2006.02.007. PIHOSH Y, BIEDERMAN H, SLAVINSKA D, KOUSAL J, CHOUKOUROV A, TRCHOVA M, MACKOVA A AND BOLDYRYEVA A (2006b), ‘Composite SiOx/hydrocarbon plasma polymer films prepared by RF magnetron sputtering of SiO2 and polyethylene or polypropylene’, Vacuum, 81, 32–37, doi: 10.1016/j.vacuum.2006.02.006. SCHOLZ J, NOCKE G AND HOLLSTEIN F (2005), ‘Investigations on fabrics coated with precious metals using the magnetron sputter technique with regard to their anti-microbial properties’, Surface and Coatings Technology, 192, 252–256, doi: 10.1016/ j.surfcoat.2004.05.036. SICHA J, MUSIL J, MEISSNER M AND CERSTVY R (2008), ‘Nanostructure of photocatalytic TiO2 films sputtered at temperatures below 200 ºC’, Applied Surface Science, 254, 3793– 3800, doi: 10.1016/j.apsusc.2007.12.003. TERAI H, KAWAKAMI A AND WANG Z (2002), ‘Sub-micron NbN/AlN/NbN tunnel junction with high critical current density’, Physica C: Superconductivity, 372–376, Part 1, 38–4, doi: 10.1016/S0921-4534(02)00700-1. THOULESS M D (1998), ‘An analysis of spalling in the microscratch test’, Engineering Fracture Mechanics, 61, 75–81, doi: 10.1016/S0013-7944(98)00049-6. UDDIN M J, CESANO F, BONINO F, BORDIGA S, SPOTO G, SCARANO D AND ZECCHINA A (2007), ‘Photoactive TiO2 films on cellulose fibres: synthesis and characterization’, Journal of Photochemistry and Photobiology A: Chemistry, 189, 286–294, doi: 10.1016/ j.jphotochem.2007.02.015. WANG H B, WANG J Y, WEI Q F, HONG J H AND ZHAO X Y (2007a), ‘Nanostructured antibacterial silver deposited on polypropylene nonwovens’, Surface Review and Letters, 14, 553–557, doi: 10.1142/S0218625X07009839. WANG Y Y, WEI Q F, LI Q AND WANG J (2007b), ‘The analysis of microstrcucture of indium tin oxide thin films deposited on polyester spunbonded nonwovens’, Materials Review, 21, 72–74. WEI Q F, WANG X Q AND GAO W D (2004), ‘AFM and ESEM characterisation of functionally nanostructured fibres’, Applied Surface Science, 236, 456–460, doi: 10.1016/ j.apsusc.2004.05.094. WEI Q F, HUANG F L, HOU D Y AND WANG Y Y (2006a), ‘Surface functionalisation of polymer nanofibres by sputter coating of titanium dioxide’, Applied Surface Science, 252, 7874–7877, doi: 10.1016/j.apsusc.2005.09.074. WEI Q F, LI Q, HOU D Y, YANG Z T AND GAO W D (2006b), ‘Surface characterization of functional nanostructures sputtered on fiber substrates’, Surface and Coatings Technology, 201, 1821–1826, doi: 10.1016/j.surfcoat.2006.03.007. WEI Q F, WANG Y Y, WANG X Q AND HUANG F L (2007a), ‘Surface nanaostructure evolution of functionalized polypropylene fibers’, Journal of Applied Polymer Science, 106, 1043–1247, doi: 10.1002/app.25401 WEI Q F, YU L Y, HOU D Y AND WANG Y Y (2007b), ‘Comparative studies of functional nanostructures sputtered on polypropylene nonwovens’, E-polymers, No.039. WEI Q F, YU L Y, MATHER R R AND WANG X Q (2007c), ‘Preparation and characterization of titanium dioxide nanocomposite fibers’, Journal of Materials Science, 42, 8001–8005, doi: 10.1007/s10853-007-1582-1. WEI Q F, TAO D, DU Z F, CAI Y B, WU N AND CHEN L (2008a), ‘Surface nanostructures and

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dynamic contact angles of functionalized poly(ethylene terephthalate) fibers’, Journal of Applied Polymer Science, 109, 654–658, doi: 10.1002/app.28073. WEI Q F, XU Q X, SHAO D F AND CAI Y B (2008b), ‘Evaluation of the interfacial bonding between fibrous substrate and sputter coated copper’, Surface and Coatings Technology, 202, 4673–4680, doi: 10.1016/j.surfcoat.2008.03.037. WILMERT D B AND HUGO L (1999), ‘Advances in magnetron sputter sources’, Thin Solid Films, 351,15–20, doi: 10.1016/S0040-6090(99)00149-2. WINDOW B (1995), ‘Recent advances in sputter deposition’, Surface and Coatings Technology, 71, 93–97, doi: 10.1016/0257-8972(95)80024-7. WONG K K H, ZINKE-ALLMANG M AND WAN W K (2006), ‘N+ surface doping on nanoscale polymer fabrics via ion implantation’, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 249, 362–365, doi: 10.1016/j.nimb.2006.04.029. XU Y, WEI Q F, WANG Y Y AND HUANG F L (2007), ‘Preparation of TiO2 coated on fabrics and their photocatalytic reactivity’, Journal of Donghua University, 24, 333–336. YASUHIKO H, MASAHITO T Y AND SETSUJI T (1998), ‘Evaluation of photocatalytic sterilization rates of Escherichia cells in titanium dioxide slurry irradiated with various light sources’, Journal of Chemical Engineering of Japan, 31, 577–584.

4 Surface grafting of textiles N. ABIDI Texas Tech University, USA

Abstract: Surface grafting of textiles is relatively recent technology that offers a variety of ways in which to alter the surface of textile substrates and, thus, impart new or improved functional properties. This chapter discusses the four main techniques for achieving surface grafting of textiles: (a) chemical graft polymerization; (b) radiation induced grafting; (c) plasmainduced grafting; and (d) light-induced grafting. The properties imparted to the surface and the application areas of the grafted surface are discussed. Strengths and weakness of each technique are reviewed. Key words: surface grafting, plasma polymerization, initiator, photopolymerization, irradiation.

4.1

Introduction

The surface composition and structure of a textile material plays an important part in the textile’s performance in a specific application. The surface of the textile material is indeed the interface or reaction region between the textile and the environment in which the material is used (Castner, 1998). As an illustration, the surface of a textile garment that is in contact with the human skin could be modified to absorb the body moisture while the outside surface could be modified to repel water. A review of literature shows the considerable attention that has been focused on the surface modification of polymeric materials. Several studies demonstrated that the properties of polymeric substrates could be altered through surface modification. Surface grafting of textiles is relatively recent technology that offers a variety of ways in which to alter the surface of textile substrates and impart new or improved functional properties. In this chapter a review of the state of the art of the current knowledge of surface grafting of textiles is presented.

4.2

Techniques of surface grafting

There are in principle four techniques for achieving surface grafting of textiles: (a) chemical graft polymerization which involves the use of initiators (e.g. ceric ion); (b) radiation-induced grafting which involves the use of high-energy radiation (e.g. γ-Co60 rays); (c) plasma-induced grafting which consists of using either radiofrequency or microwave plasma; and (d) light-induced grafting which 91

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involves using a source of ultraviolet radiation. These four methods share the same objectives, creating free radical sites within the macromolecules of the substrate. Then, these radical sites are used as initiators for copolymerization reactions with vinyl monomers present in the grafting solution. Parameters such as concentration of the monomer, time of treatment, type and concentration of the catalysts, and dose of radiation could greatly affect the grafting efficiency and need to be optimized for maximum yield.

4.2.1

Chemical grafting

In the chemical grafting process, the first and most important step is to create radicals on the textile substrate, which will be used to initiate copolymerization reactions with different monomers. Various initiators are used: potassium permanganate (Deo and Gotmare, 1999); ammonium peroxydisulfate (Tsukada et al., 1997); ceric ion (Sharma and Daruwalla, 1976; Zahran, 2006); benzoyl peroxide (Louati et al., 1999; Celik, 2004); semiconductor-based photocatalyst (Ojah and Dolui, 2007); 2,2′-azobis-(2-methylpropionamideine) (Liu and Sun, 2006); ammonium persulfate (Kawahara et al., 1997); and hydrogen peroxide (Hebeish et al., 1981). Deo and Gotmare (1999), in a research work aimed at grafting acrylonitrile (AN) monomer on gray cotton to impart high water absorbency, used a KMnO4HNO3 as a redox system. The authors tentatively provided a mechanism for the KMnO4-induced grafting. They hypothesized that in the presence of an acid, under the catalytic influence of KMnO4, radical sites are created within the cellulose backbone (Deo and Gotmare, 1999). Then, in the presence of vinyl monomer, these radicals initiate copolymerization reactions. The results obtained allowed the authors to conclude that for optimal grafting (about 37.76%), the concentration of KMnO4 should be maintained at 0.05 M, the acid concentration (HNO3) maintained at 0.08 M, the monomer concentration maintained at 6.4% (w/v), the material:liquid ratio maintained at 1:100, the temperature of the grafting solution maintained at 60 ºC, and the reaction time maintained at 60 min. Ammonium peroxydisulfate was used as initiator to graft benzyl methacrylate onto wool fibers (Tsukada et al., 1997). The results reported showed that poly (benzyl methacrylate) was successfully grafted on wool fibers. The authors reported that the tensile strength of grafted wool fibers increased when the add-on was in the range 45–77% add-on, while the elongation at break decreased. In addition, measurements using differential scanning calorimetry and thermogravimetric analysis revealed a higher thermal stability of the grafted wool fibers when compared with untreated fibers (Tsukada et al., 1997). Methacrylic acid and other vinyl monomers were grafted onto the surface of cotton fabric using a tetravalent ceric ion (CeIV)–cellulose thiocarbonate redox

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93

system (Zahran, 2006). This system has shown to be efficient at creating macroradicals within the cellulose macromolecules. The author investigated the effect of different parameters (such as pH of the monomer solution, initiator and monomer concentration, time and temperature of the grafting, and type of monomer). The results obtained showed that a graft yield of 45% of methacrylic acid could be achieved by maintaining the pH at 2, the concentration of the initiator (Ce(SO4)2) at 20 mmol l–1, the concentration of methacrylic acid at 4%, and the temperature of the grafting solution at 60 ºC. In order to impart water repellency, chemical/thermal stability, and enhance adhesion, polyester fibers were grafted with perfluorooctyl-2 ethanol acrylic monomer (Louati et al., 1999). In this study, the authors used benzoyl peroxide as initiator. Keeping the concentration of bezoyl peroxide at 1.8 × 10–2 mol l–1, the temperature of the grafting solution at 85 ºC, the reaction time at 1 h, and the material:liquid ratio at 1:100; a maximum grafting yield was obtained at a monomer concentration of 0.125 mol l–1. A decrease in the grafted amount was observed beyond this concentration, which was attributed to the occurrence of homopolymerization reactions with increasing monomer concentration. A similar observation was made by Abidi and Hequet (2004). Methacrylamide was graft-copolymerized on acrylic fibers using benzoyl peroxide as initiator (Celik, 2004). The effects of the concentration of the initiator, the concentration of the monomer, the temperature of the grafting solution, and the duration of the grafting on the percentage grafting were investigated. It was found that for fixed initiator concentrations (3 × 10–3 mol l–1), fixed monomer concentration (0.39 mol l–1), and fixed reaction time (180 min), the maximum grafting percentage was achieved at a grafting solution temperature of 85 ºC (see Fig. 4.1). Furthermore, the author reported that for maximum grafting yield the concentration of the initiator should be around 3 × 10–3 mol l–1. The effect of the concentration of the monomer was also shown to be critical to maximizing the grafting yield (see Fig. 4.2). Recently, it was reported that a semiconductor-based photocatalyst (cadmium sulfide, CdS) could be used to initiate graft-copolymerization reactions of methyl methacrylate and acrylamide monomers on silk fibers (Ojah and Dolui, 2007). In this experiment, silk fibers were placed in the grafting solution composed of the monomer, the photocatalyst (CdS), and triethylamine or ethylene glycol, and then exposed to ultraviolet radiations (wavelengths in the range 500–600 nm) for different periods of time and temperatures. Using this approach, 10–48% grafting yield was achieved with methyl methacrylate and 4–26% grafting yield with acrylamide (Ojah and Dolui, 2007). The authors concluded that, when used in combination with additives, a semiconductor-based photocatalyst appears to be a promising initiator for the graft copolymerization of methyl methacrylate and acrylamide onto silk fibers. Grafted silk fibers exhibited increased chemical resistance, increased elongation at break, decreased slopes of stress–strain curves, and decreased water retention (Ojah and Dolui, 2007).

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120 85 °C 100

˜ ◊

˜

˜

95 °C ◊

◊

Graft yield (%)

80 ˜ ◊

60 ◊ ¤

◊

40

˜

◊

¤

¤

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20 ◊

0

80 °C ¤

◊ ˜◊ ˜ ˜˜

0

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¤

¤







40

75 °C





80 120 Time (min)







160

200

4.1 Evolution of the graft yield (expressed in percent) as a function of the temperature of the grafting solution and the reaction time. Concentration of benzoyl peroxide, 3.0 ×10–3 mol l–1; concentration of the monomer, 0.39 mol l–1 (reprinted with the permission of John Wiley & Sons, Inc. from Celik (2004)).

4.2.2

Radiation-induced grafting

Radiation-induced grafting involves the use of high-energy radiation (e.g. γ-Co60). This could be carried out by the following methods: (a) simultaneous irradiation and grafting through in-situ-formed free radicals; (b) grafting through peroxide groups introduced by pre-irradiation; and (c) grafting initiated by trapped radicals formed by pre-irradiation (Uyama et al., 1998). Compared with chemical grafting, radiation grafting has many advantages such as the fact that no chemical initiators are used and that grafting yield can be controlled by controlling irradiation conditions (e.g. irradiation dosages and irradiation time). Cellulose (originating from viscose rayon) was graft-copolymerized with styrene (Kobayashi, 1961). In this experiment, cellulose was irradiated by γ-Co60 rays in water or in an H2O2 solution for 24 h at a dose rate of 3.65 × 104 roentgen/h. The degree of grafting was influenced by the contact time between cellulose and styrene. The number of active sites produced during the irradiation was estimated

Surface grafting of textiles

95

200

0.78 M ˜

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160

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0.59 M ◊

Graft yield (%)

◊ 120

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0.20 M


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40 ˜

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0.39 M ¤

¤

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Time (min)

4.2 Evolution of the graft yield (expressed in percent) as a function of the concentration of the monomer and the reaction time. Concentration of benzoyl peroxide, 3.0 × 10–3 mol l–1 (reprinted with the permission of John Wiley & Sons, Inc. from Celik (2004)).

by the ferrous ion method, which consists of determining the number of moles of hydroperoxide per 1000 glucose mole units (Kobayashi, 1961). It was shown that the number of active sites increases with increasing radiation time until it reaches a plateau at around 40 h of irradiation (Kobayashi, 1961). Huang et al. (1963) reported on the radiation-induced grafting of vinyl monomers on cellulose (purified cotton linters and cotton cloth). The authors explored the effect of the γ-Co60 radiation on the structure of cellulose in cotton. They reported that the exposure of cotton to a total radiation dose of 2 × 106 roentgen resulted in a 77% decrease in the degree of polymerization of the cellulose. This indicated that

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if the conditions are not optimized during the γ-Co60 irradiation process, serious degradation of the textile substrate could occur (Huang et al., 1963). Shao et al. (2001) used an electron beam to induce grafting of 2-hydroxyethyl methacrylate (HEMA) on silk fabric. The electron beam equipment used was a GJ-1.5 high-frequency and high-pressure electric accelerator having a maximum power of 1.5 MeV and a maximum intensity of electron beam of 1.5 mA (Shao et al., 2001). The results showed that the degree of grafting was affected by the absorbed dose and the irradiation method (pre-irradiation grafting or coirradiation grafting). The co-irradiation method, where silk was treated with HEMA prior to its exposure to electron beam irradiation, yielded a higher degree of grafting (Shao et al., 2001). The authors showed that the physical and mechanical properties of silk fabrics were impacted by electron beam irradiation-induced grafting of HEMA. Silk fabrics were also subjected to two treatments: irradiation grafting with methacrylamide and irradiation cross-linking with dimethyloldihydroxyethylene (Liu et al., 2004). The results showed that while the radiation grafting with methacrylamide increased silk weight, radiation cross-linking improved the resistance of silk fabric to the formation of wrinkles. Acrylic acid was grafted onto polyester fibers by means of γ-Co60 irradiation followed by treatment with chitosan and collagen (Hu et al., 2002; Jou et al., 2007). The pre-irradiated fibers, at a total irradiation rate of 10 kGy, were placed in 10 wt% acrylic acid aqueous solution containing 0.2 M H2SO4 and 0.001 M FeSO4. This led to the introduction of functional carboxyl groups on the polyester fiber surface. Subsequently, chitosan was grafted by means of an esterification reaction followed by collagen immobilization. The results showed that the resulting material exhibits antibacterial activity against four pathogenic bacteria: Staphylococcus aureus-1, Escherichia coli, Pseudomonas aeruginosa and S. aureus-2 (Jou et al., 2007).

4.2.3

Plasma-induced grafting

Plasma technology is another method used for textile surface modification (Yasuda et al., 1984; Bhat and Benjamin, 1999; Wong et al., 1999; Radetic et al., 2000; Abidi and Hequet, 2004; Abidi and Hequet, 2005). This technique is based on the use of activated species (such as ions, radicals, metastable molecules) to create active sites within the structure of the molecules of the substrates, which could be used to initiate copolymerization reactions with different monomers. Abidi and Hequet (2004) reported on the use of O2, N2, and Ar plasmas to generate active sites on the surface of the cotton fabric. The authors showed that exposure of cotton fabric to microwave plasma (2.45 GHz, 500 W) for 240 s produced vibration bands at 1725 cm–1 in the Fourier transform infrared (FTIR) spectra of the treated fabrics. This band was attributed to asymmetric stretching of the carbonyl band. It was reported in the same study that Ar plasma generated more

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active sites than O2 and N2 plasmas. The integrated intensity of the peak at 1725 cm–1 (I1725) increased both with increasing microwave power and treatment time, t (I1725 (Ar) = 0.002 + 0.0002 × power, r = 0.985; I1725 (Ar) = 0.021 + 0.0003t, r = 0.979). Ward et al. (1979) reported that the exposure of cotton fabric to Arradiofrequency plasma for 30 min (power of 40 W) generated a vibration band at 1724 cm–1 in the FTIR spectra of the treated fabric. Most of the active sites created by the plasma irradiation, when the plasmairradiated substrate is exposed to air, could recombine or convert to peroxides. It was reported that the peroxide concentration is almost equivalent to the concentration of the radicals created by the plasma (Inagaki, 1996). Abidi and Hequet (2004) reported that the integrated intensity of the peak at 1725 cm–1 created by exposure to microwave plasma decreased when the treated fabric was exposed to ambient conditions. For Ar plasma, a linear decrease with post-plasma time was observed: I1725 = 0.0738 – 0.0011t, r = –0.919. Therefore, the plasma-activated fabric should be quickly reacted with the monomer. The effect of plasma on the weight loss and strength of the cotton fabric has been investigated (Abidi and Hequet, 2005). It was found that the exposure of cotton fabric to microwave Ar plasma for 240 s (microwave power, 500 W) resulted in less than 0.4% weight loss, while for O2 plasma weight loss was 1.6%. Wong et al. (1999) reported that exposure of linen fabric to radiofrequency plasma at 200 W for 60 min resulted in 10.34% weight loss with O2 plasma and 1.69% weight loss with Ar plasma. Exposure of cotton fabric to O2 plasma generated higher weight loss and had a negative impact on the breaking strength of the fabric (Abidi and Hequet, 2005). Vinyl laurate monomer (CH3(CH2)10–COO–CH=CH2) was grafted onto a cotton fabric surface (Abidi and Hequet, 2004). For this purpose, the fabric was exposed to microwave plasma for a specified period of time. Then, the fabric was taken out of the plasma chamber and immediately immersed in the monomer solution. After rinsing, drying, and conditioning the FTIR spectra were recorded (see Fig. 4.3). The presence of additional peaks located at 1735 cm–1 (–C=O stretching vibration), 2855 cm–1 (–CH2 symmetric stretching vibration), and 2923 cm–1 (–CH2 asymmetrical stretching vibration) indicated the efficiency of plasmainduced grafting of vinyl laurate monomers on the fabric surface. The amount of grafted monomer on the fabric surface was calculated and reported as a function of the monomer concentration in the grafting solution. The results showed an increase in the grafted amount with increasing monomer concentration in the grafting solution until it reached a maximum at 0.664 mol l–1, and then it decreased (see Fig. 4.4). This behavior was attributed to the fact that the increase in the monomer concentration increased the probability of homopolymer formation rather than grafting with the cellulose macromolecules (Abidi and Hequet, 2004). Radiofrequency plasma was used to graft acrylamide onto cotton and polyester fabrics (Bhat et al., 1999). Fabrics were first immersed in aqueous acrylamide

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2923 1735 2855

Absorbance

c

b a

3600

3400

3200

3000

2800 1800

1600

1400

Wavenumber (cm–1)

4.3 FTIR spectra of: (a) control; (b) plasma-induced grafting of vinyl laurate on the cotton fabric; (c) plasma-induced grafting of vinyl laurate on the cotton fabric after ten home laundering cycles (reprinted with the permission of John Wiley & Sons, Inc. from Abidi and Hequet (2005)).

õ

1000

õ

Grafted amount (µg cm–2)

õ

800 õ

õ

600

400

õ õ

200 õ

0 0.0

õ

0.1

0.2

0.3

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0.5

0.6

0.7

0.8

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–1

Vinyl laurate concentration (mol l )

4.4 Variation of the amount of grafted monomer (expressed in µg cm–2) as a function of the monomer concentration (reprinted with the permission of John Wiley & Sons, Inc. from Abidi and Hequet (2004)).

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solutions of various concentrations, air dried, and then exposed to radio frequency plasma. The results indicated that the surface resistivity of cotton was lowered and that of polyester was drastically lowered after grafting (Bhat et al., 1999). Tsafack and Levalois-Grützmacher (2006) used a low-pressure Ar-plasma process to graft acrylate monomers containing phosphorous onto polyacrylonitrile textiles. The authors investigated plasma-induced grafting of four acrylate monomers: diethyl(acryloyloxyethyl)-phosphate, diethyl-2-(methacryloyloxyethyl)phosphate, diethyl(acryloyloxymethyl)phosphonate, and dimethyl(acryloyloxymethyl)phosphonate. These monomers are known to exhibit flameproof properties when applied to polymeric substrates using the classical polymerization reactions. The flame retardancy of the resulting copolymers was assessed by measuring the limiting oxygen index and the thermal stability was studied by thermogravimetric analysis. The authors showed that compared with ungrafted polyacrylonitrile fabric (which starts burning at 18% of oxygen), plasma-grafted fabrics started burning at oxygen concentrations of 21% and higher depending on the type of monomer used and its concentration. The authors concluded that the microwave plasma-induced graft polymerization can seriously compete with traditional chemical treatments using organo-phosphorous compounds. Gawish et al. (2007) reported that new functionalities could be added to polyamide 6,6 fabrics by means of atmospheric plasma. The atmospheric pressure plasma device that was used consisted of a capacitively coupled dielectric barrier discharge which was operated by a 4.8 kW audio frequency power supply at 4–10 kHz. Successful plasma-induced grafting of glycidyl methacrylate on polyamide 6,6 was reported. These grafted fabrics were then used to link triethylene tetramine, quaternary ammonium chitosan, and β-cycoldextrin (Gawish et al., 2007). Graft polymerization of hydrophilic monomers onto textile fibers was performed by means of radiofrequency plasma (Zubaidi and Hirotsu, 1996). The textile fibers used were cotton, cellulose acetate, rayon, and cupraamonium cellulose. The monomers used were HEMA, acrylamide, N-isopropyl acrylamide, acrylic acid, 2-methoxyethyl acrylate, and 2-hydroxyethyl methacrylate. The results obtained showed that the amount of monomers grafted was dependent on the type of textile substrate (Zubaidi and Hirotsu, 1996). Cotton fibers were shown to be more reactive than other fibers. In addition, the authors reported that the HEMA monomer was graft polymerized more readily than the other monomers investigated. Polyester fabric was treated with low-temperature plasma to impart soil resistance and improve dyeability (Oktem et al., 1999). The authors incorporated acrylic acid on the surface of polyester fabrics using two alternative plasma treatment procedures. In the first procedure, in-situ polymerization of acrylic acid monomers was achieved in a glow discharge reactor. In the second approach, the

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fabric was first treated in Ar plasma, followed by immersion in an aqueous bath containing different amounts of acrylic monomers (Oktem et al., 1999). Cotton fabric was activated using continuous Ar-radiofrequency plasma for subsequent surface-initiated grafting reactions and/or graft polymerization with different functional methacrylate monomers (Castelvetro et al., 2006). The authors reported that functional acrylic monomers could be efficiently grafted onto mercerized cotton fabrics by means of continuous Ar plasma. Two experiments were adopted: a one-step process, in which the fabric initially impregnated with the monomer was placed in the plasma chamber and then exposed to Ar plasma; and a two-step process in which the fabric was first exposed to plasma for a specified time and then immediately immersed in the monomer solution outside the plasma chamber (Castelvetro et al., 2006). The maximum grafting yield was obtained when the fabric was exposed to plasma generated using a 200 W radio frequency generator (exposure time was constant at 180 s). Nonwoven polypropylene fabrics were modified using atmospheric plasma in order to impart biocidal functions (Gawish et al., 2007; Wafa et al., 2007). Atmospheric audio frequency plasma (4.8 kW audio frequency power supply in the frequency range of 5–10 KHz) was used to activate the polypropylene surface followed by graft copolymerization of glycidyl methacrylate (GMA). The results obtained showed an effect of the plasma exposure time on the percentage add-on of GMA on the fabric surface. The grafted polypropylene-GMA epoxide group was reacted with different compounds: β-cyclodextrin, monochlorotrizynyl-βcyclodetrins, or N-(2 hydroxy propyl)3-trimethylammonium chitosan chloride (Gawish et al., 2007). Hochart et al. (2003) reported that a good and durable water and oil repellency could be imparted to polyacrylonitrile (PAN) fabrics through plasma-induced grafting of 1,1,2,2, tetrahydroflurodecyl acrylate (Hochart et al., 2003). PAN fabrics were exposed to microwave Ar plasma for 10 min (microwave generator power, 100 W flow rate; 0.5 l min–1), immersed in the monomer solution, and exposed again to the plasma using the same conditions.

4.2.4

Light-induced grafting

Ultraviolet radiation-induced surface grafting of textiles is another technique that has attracted attention because of its simplicity. In this process, textile substrates are exposed to ultraviolet radiation (hν) which creates radicals within the molecules constituting the substrate. These radicals are then used to initiate copolymerization reactions with various monomers. Because the ultraviolet radiation is not as penetrating as high-energy radiation (γ-rays), free radicals are produced mainly close to the surface rather than uniformly distributed throughout the fibers (Reinhardt and Harris, 1980). Ferrero et al. (2008) reported on the use of ultraviolet curing of silicone and urethane acrylates with different formulations on cotton fabrics in order to impart

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water repellency. In this experiment, the formulation was first prepared and the fabric was coated and allowed to dry to remove the solvents. The surface-coated fabrics were then exposed to ultraviolet radiation. The results obtained showed strong hydrophobic behavior (Ferrero et al., 2008). Acrylamide monomers were photo-induced graft-copolymerized onto polyamide-6 (PA-6) (Bogoeva-Gaceva et al., 1993). In this experiment, the authors pretreated PA-6 fibers at room temperature for 30 s with a benzophenone solution in a mixture of CH2Cl2 and CH3OH, followed by irradiation in a quartz cuvette containing an aqueous solution of monomer. The irradiation source was a 500 W high-pressure Hg lamp and the distance between the lamp and the sample was set to 6 cm (Bogoeva-Gaceva et al., 1993). The ultraviolet-induced graft exhibited good hydrophilicity. Ultraviolet-induced grafting of a water-soluble monomer (such as acrylamide, poly(ethylene glycol) methacrylate, 2-acrylamide-2-methyl propane sulfonic acid, dimethyl aminoethyl methacrylate) on the surface of poly(ethylene terephtalate) (PET) fabrics was performed to permanently change the surface properties from hydrophobic to hydrophilic (Uchida et al., 1991). The objective of the study was to improve the antistatic properties of PET textiles. The authors reported that the PET fabric, along with the aqueous monomer solution and NaIO4, was placed in a Pyrex glass and exposed to ultraviolet radiation (λ > 250 nm) for 90 min at 30 ºC without degassing. When the concentration of the monomers was kept at 10 wt% and the concentration of NaIO4 at 5 × 10–4 M, the amount of graft on the fabric surface was 0.39% for the acrylamide monomer and 2.55% for the dimethyl aminoethyl methacrylate monomer. However, for 2-acrylamide-2-methyl propane sulfonic acid and poly(ethylene glycol) methacrylate monomers, the authors reported that the change in weight was too small to be measured using their balance (Uchida et al., 1991). Polypropylene staple fibers were grafted using ultraviolet radiation with HEMA in the presence of three different photoinitiators: uranyl nitrate, ceric ammonium nitrate, and benzoin ethyl ether (Shukla and Athalye, 1994). The choice of grafting with HEMA was based on its hydrophilic properties. When grafted on polypropylene fabrics, the moisture regain of the fabric was increased and the build-up of static electricity was diminished. The parameters affecting the efficiency of the grafting, such as time and temperature of the reaction as well as concentrations of the initiators, were optimized in this study to maximize the amount of grafted monomer on the fabric surface.

4.3

Properties achieved and applications

Surface grafting of textile substrates imparts new functionalities to the resulting materials. Acrylonitrile monomer grafted on cotton fibers imparts high water absorbency (Deo and Gotmare, 1999; Hochart et al., 2003). The moisture content and water absorbency of acrylic fibers are highly enhanced by graft

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Contact angle (degrees)

140 120 100 80 60 40 20 0 Control

A

B

4.5 Contact angle measurements of: a control untreated cotton fabric; a cotton fabric grafted with vinyl laurate monomer (A), and a cotton fabric grafted with vinyl laurate monomer and subjected to eight cycles of home laundering and tumble drying (B).

copolymerization of methacrylamide (Celik, 2004). The development of materials with high absorbing capacities for water or other liquids is of particular importance. Considerable interest in recent years has been given to modified polymers that could be used in disposable diapers, sanitary napkins, bandages etc. Plasma-induced grafting of vinyl laurate monomer onto lightweight cotton fabric imparts very good water repellency (Abidi and Hequet, 2004). As an illustration, Fig. 4.5 shows the water contact angles of: a control cotton fabric (<5º); a cotton fabric that was subjected to plasma-induced grafting of vinyl laurate (119º); and a cotton fabric that was subjected to plasma-induced grafting of vinyl laurate followed by eight cycles of home laundering and tumble drying (121º). A water contact angle of 119º indicates excellent water repellency of the grafted fabric. Furthermore, the fact that the water contact angle remains unchanged after repeated home laundering signals the establishment of covalent bonds between cellulose macromolecules and vinyl monomers. The water and oil repellency of polyester fibers could be greatly improved by chemical-induced grafting of perfluorooctyl-2 ethanol acrylic monomer (Louati et al., 1999). By introducing fluorine into polymeric chains, the textile substrate could be modified in order to provide low surface energy as well as thermal and chemical stability. Fluorine monomers form a protective layer on the surface, which decreases the possibility of spreading and diffusion of liquids inside the fibers. PAN fibers have wide use in home furnishing, for example furniture coverings, curtain materials, wall hangings, textile wall coverings, carpets, and knitwear clothing. Therefore, imparting flame resistance to these fibers is of particular importance. Fire-retardant monomers were graft-copolomerized using microwave plasma (Tsafack and Levalois-Grutzmacher, 2006). Using this approach, fire retardancy was successfully imparted to polyacrylonitrile textiles.

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Grafting wool fibers with benzyl methacrylate was reported to lead to structural and physico-mechanical changes without affecting the intrinsic properties of the fibers (Tsukada et al., 1997). Among the changes, the authors reported that the grafted wool fibers exhibited higher thermal stability. The graft copolymerization of vinyl monomers onto silk results in increased thermal stability, increased chemical resistance, increased elongation at break, and increased water retention values (Ojah and Dolui, 2007). PET could be used in biomedical applications such artificial vascular, laryngeal, esophageal, and suture materials (Jou et al., 2007). The surface modification of the PET becomes important when PET substrates are in contact with blood or living tissues. The grafting of polyester fibers with acrylic acid to introduce functional groups followed by grafting of chitosan and collagen lead to the formation of a copolymer that has a good antibacterial activity against pathogenic bacteria (Jou et al., 2007). The phenomenon of surface resistivity is described as the propensity of textile fibers to generate static electricity because of their hydrophobic nature. Plasmatreated and plasma-grafted cotton and polyester fabrics exhibit a dramatic decrease in their surface resistivity (Bhat and Benjamin, 1999). The decrease in surface resistivity is the result of an increase in moisture retention on the surface because of the creation of polar groups (which enhance surface conductivity) and the changes in the surface morphology owing to the etching action. Ultravioletinduced graft polymerization of water-soluble monomers can permanently change the hydrophobic surface of polyester fabrics to one that is hydrophilic (Uchida et al., 1991). This confers antistatic properties to polyesters, as shown by measuring the triboelectrostatic potential generated upon rubbing against a cotton fabric, the decay time of the electrostatic potential, and the surface electrical resistance of the fabric (Uchida et al., 1991). Areas of application for these grafted surfaces vary depending on the type of monomer being grafted on the surface of the textile substrates. As mentioned above, textile substrates could be grafted to impart the following properties: • Water repellency for outdoor applications. Textile materials treated to impart this property have many applications; for example, clothing, outdoor awnings, car tops, umbrellas, and outdoor furnishings. • Water, oil, and stain repellency for use in textiles designed for daily wear – for curtains, napkins, etc. In addition, because the water and oil do not diffuse inside the fiber, laundering and drying would take less time – leading to big savings in water and energy costs. Textile fabrics that have been treated for stain repellency and soil release are particularly suitable for active wear and leisure wear (Schindler and Hauser, 2004). The market for this type of clothing is growing. Industrial uniforms and napery are other market areas where soil release is a desirable product feature (Holme, 1993). • Antibacterial textiles for medical applications. Textile materials that are treated

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to withstand microbial growth are of particular importance in areas where contamination from pathogens can occur (e.g. hospitals, nursing homes, hotels). • Industrial textiles for applications in the automotive and airline industries. Water-oil-stain-repellent, antimicrobial, and flame retardant properties are of particular interest. • Protective clothing with antistatic properties that prevent the build-up of static electricity and provide comfort to the wearer. When the surface of a textile material is modified with antistatic finishing agents, the conductivity of the textile material is increased (the surface resistivity is lowered). Thus, the surface charge build-up is reduced and rapid dissipation of electrical charges is achieved.

4.4

Strengths and weaknesses of surface grafting

For many years, the method used to graft monomers onto textile substrates was based on the use of catalysts (such as ceric ion) as initiators of the copolymerization between the molecules of the substrate and the monomer. The chief drawback of these methods is the generation of highly toxic wastes. In addition, the modification is not limited solely to the surface of the material. Radiation grafting using high-energy sources such as ultraviolet radiation or electron beams has been explored as an alternative to chemical grafting. However, if the conditions are not controlled, radiation grafting may result in deterioration of the physical properties of the substrate and may also lead to the photo-degradation of the substrate. Furthermore, when using γ-irradiation, dosing should be very limited and controlled because of the high penetrability of the γ-rays into the textile substrate which can cause severe breakage of the chemical bonds. Plasma-induced grafting has emerged as an alternative and attractive method for surface modifications of textile substrates. The plasma technology is composed of three different processes. The first is modification of the surface structure of the material itself, under the influence of the glow discharge; this is done primarily with non-polymerizable precursors such as noble gases, nitrogen, oxygen, hydrogen, ammonia, or water vapor. The second process is deposition of a thin film of polymer on the surface of the material by means of organic, organosilicone, or organo-metallic vapors; this is referred to as plasma polymerization. The third stage is the grafting of monomers onto the surface of the material after plasma activation; this is referred to as plasma grafting. The advantage of the plasma treatment over the wet chemistry technique is that the effects of the plasma do not penetrate more than 100 Å from the surface. Since the bulk of the material is not affected by the treatment, desirable structural characteristics are maintained. Furthermore, the plasma process does not generate toxic wastes (catalysts).

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4.5

105

Future trends

From a literature review, it is clear that more research is being conducted on plasma-induced grafting. This demonstrates that plasma technology has the potential to become the technology of choice for surface modification and grafting. Current investigations are focused mainly on developing atmospheric, roll-to-roll plasma treatment for continuous operation in textile mills. A novel surface modification approach that is currently being investigated in the author’s group is atomic layer deposition. This approach allows us to perform nanocoating deposition on the fabric surface to impart various interesting properties; preliminary results are very encouraging.

4.6

Sources of further information and advice

CASTNER D G (1998), Chemical modification of surfaces. In Specimen Handling, Preparation, and Treatment in Surface Characterization. Edited by Czanderna A.W, Powell C J and Madey T E, Kluwer Academic/Plenum Publishers, New York, pp. 209–238. HOLLAHAN J R AND BELL A T (1974), Techniques and Applications of Plasma Chemistry, John Wiley & Sons, New York. INAGAKI N (1996), Plasma Surface Modification and Plasma Polymerization. Technomic, Lancaster, Pennsylvania. SCHINDLER W D AND HAUSER P J (2004), Chemical Finishing of Textiles. Woodhead Publishing Ltd, Cambridge, UK. UYAMA Y, KATO K AND IKADA Y (1998), ‘Surface Modification of Polymers by Grafting’, Adv Polym Sci, 137, 1–39.

4.7

References

ABIDI N AND HEQUET E (2004), ‘Cotton Graft Copolymerization Using Microwave Plasma. I. Universal Attenuated Total Reflectance-FTIR Study’, J Appl Polym Sci, 93, 145–154. ABIDI N AND HEQUET E (2005), ‘Cotton Graft Copolymerization Using Microwave Plasma. II. Physical Properties’, J Appl Poly Sci, 98, 896–902. BHAT N V AND BENJAMIN Y N (1999), ‘Surface Resistivity Behavior of Plasma Treated and Plasma Grafted Cotton and Polyester Fabrics’, Textile Res J, 69(1), 38–42. BOGOEVA-GACEVA G, PIMONENKO N Y AND PETROV G (1993), ‘Photo-Induced Acrylamide Graft Polymerization onto Polyamide-6’, Textile Res J, 63(1), 51–57. CASTELVETRO V, FATARELLA E, CORSI L, GIAIACOPI S AND CIARDELLI G (2006), ‘Graft Polymerization of Functional Acrylic Monomers onto Cotton Fibers Activated by Continuous Ar Plasma’, Plasma Process Polym, 3, 48–57. CASTNER D G (1998), ‘Chemical Modification of Surfaces’. In Specimen Handling, Preparation, and Treatment in Surface Characterization. Edited by Czanderna A W, Powell C J and Madey T E, Kluwer Academic/Plenum Publishers, New York, pp. 209– 238. CELIK M (2004), ‘Graft Copolymerization of Methacrylamide onto Acrylic Fibers Initiated by Benzoyl Peroxide’, J Appl Polym Sci, 94, 1519–1525. DEO H T AND GOTMARE V D (1999), ‘Acrylonitrile Monomer Grafting on Gray Cotton to Impart High Absorbency’, J Appl Polym Sci, 72, 887–894.

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FERRERO F, PERIOLATTO M, SANGERMANO M AND BIANCHETTO SONGIA M (2008), ‘WaterRepellent Finishing of Cotton Fabrics by Ultraviolet Curing’, J Appl Polym Sci, 107, 810–818. GAWISH S M, MATTHEWS S R, WAFA D M, BREIDT F AND BOURHAM M A (2007), ‘Atmospheric Plasma-Aided Biocidal Finishes for Nonwoven Polypropylene Fabrics. I. Synthesis and Characterization’, J Appl Polym Sci, 103, 1900–1910. GAWISH S M, RAMADAN A M, CORNELUIS C E, BOURHAM M A, MATTHEWS S R, MCCORD M G, WAFA D M AND BREIDT F (2007), ‘New functionalities of PA6,6 Fabric Modified by Atmospheric Pressure Plasma and Grafted Glycidyl Methacrylate’, Textile Res J, 77(2), 92–104. HEBEISH A, SHALABY S E AND BAYAZEED A M (1981), ‘Graft Polymerization of Methyl Methacrylate on Poly(ethylene Terephthalate) Fibers Using H2O2 as Initiator’, J Appl Polym Sci, 26, 3253–3269. HOCHART F, DE JAEGER R AND LEVALOIS-GRUTZMACHER J (2003), ‘Graft-Polymerization of a Hydrophobic Monomer onto PAN Textile by Low-pressure Plasma Treatments’, Surface Coat Technol, 165, 201–210. HOLME I (1993), ‘New Development in the Chemical Finishing of Textiles’, J Textile Inst, 84(4), 520–533. HU S G, JOU C H AND YANG M C (2002), ‘Surface Grafting of Polyester Fiber with Chitosan and the Antibacterial Activity of Pathogenic Bacteria’, J Appl Polym Sci, 86, 2977–2893. HUANG R Y M, IMMERGUT B, IMMERGUT E H AND RAPSON W H (1963), ‘Grafting Vinyl Polymers onto Cellulose by High Energy Radiation. I. High Energy Radiation-Induced Graft Copolymerization of Styrene onto Cellulose’, J Polym Sci: Part A, 1, 1257–1270. INAGAKI N (1996), Plasma Surface Modification and Plasma Polymerization, Technomic, Lancaster, Pennsylvania. JOU C.-H, LIN S.-M, YUN L, HWANG M.-C, YU D G, CHOU W.-L, LEE J.-S AND YANG M.-C (2007), ‘Biofunctional Properties of Polyester Fibers Grafted with Chitosan and Collagen’, Polym Adv Technol, 18, 235–239. KAWAHARA Y, SHIOYA M, KIKUTANI T AND TAKAKU A (1997), ‘Mechanical Properties and Structure of Methacrylamide (MAA) Grafted Silk Fibers’, J Text Inst, 88(1), 5–11. KOBAYASHI Y (1961), ‘Gamma-Ray-Induced Graft Copolymerization of Styrene onto Cellulose and Some Chemical Properties of the Grafted Polymer’, J Polym Sci, 51, 359– 372. LIU J, SHAO J AND ZHENG J (2004), ‘Radiation Grafting/Crosslinking of Silk Using Electron Beam Irradiation’, J Appl Polym Sci, 91, 2028–2034. LIU S AND SUN G (2006), ‘Durable and Regenerated Biocidal Polymers: Acrylic NHalamine Cotton Cellulose’, Ind Eng Chem Res, 45, 6477–6482. LOUATI M, ELACHARI A, GHENAIM A AND CAZE C (1999), ‘Graft Copolymerization of Polyester Fibers with a Fluorine-Containing Monomer’, Textile Res J, 69(5), 381–387. OJAH R AND DOLUI S K (2007), ‘Graft Copolymerization of Vinyl Monomers onto Silk Fibers Initiated by a Semiconductor-Based Photocatalyst’, J Appl Polym Sci, 105, 2164– 2175. OKTEM T, AYHAN H, SEVENTEKIN N AND PISKIN E (1999), ‘Modification of polyester fabrics by in situ plasma or post-plasma polymerization of acrylic acid’, J Society Dyers & Colorists, 115, 274–279. RADETIC M, JOCIC D, JOVANCIC P AND TRAJKOVIC R (2000), ‘The effect of low-temperature plasma pretreatment on wool printing’, AATCC Rev, 32(4), 55–60. REINHARDT R M AND HARRIS J A (1980), ‘Ultraviolet Radiation in Treatments for Imparting Functional Properties to Cotton Textiles’, Textile Res J, 50, 139–147.

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SCHINDLER W D AND HAUSER P J (2004), Chemical Finishing of Textiles. Woodhead Publishing Ltd, Cambridge, UK. SHAO J, LIU J AND CHEN Z (2001), ‘Grafting Silk with Electron Beam Irradiation’, Color Technol, 117, 229–233. SHARMA V N AND DARUWALLA E H (1976), ‘Crosslinking of Cotton through Graft Copolymerization’, Textile Res J, 46(6), 398–405. SHUKLA S R AND ATHALYE A R (1994), ‘Ultraviolet Radiation-Induced Graft Copolymerization of 2-Hydroxyethyl Methacrylate onto Polypropylene’ J Appl Polym Sci, 51, 1567–1574. TSAFACK M J AND LEVALOIS-GRÜTZMACHER J (2006), ‘Plasma-Induced Graft-polymerization of Flame Retardant Monomers onto PAN Fabrics’, Surface Coat Technol, 200, 3503–3510. TSUKADA M, SHIOZAKI H, FREDDI G AND CRIGHTON J S (1997), ‘Graft Copolymerization of Benzyl Methacrylate onto Wool Fibers’, J Appl Polym Sci, 64, 343–350. UCHIDA E, UYAMA Y AND IKADA Y (1991), ‘Antistatic Properties of Surface-Modified Polyester Fabrics’, Textile Res J, 61(8), 483–488. UYAMA Y, KATO K AND IKADA Y (1998), ‘Surface Modification of Polymers by Grafting’, Adv Polym Sci, 137, 1–39. WAFA D M, BREIDT F, GAWISH S M, MATTHEWS S R, DONOHUE K V, ROE R M AND BOURHAM M A (2007), ‘Atmospheric Plasma-Aided Biocidal Finishes for Nonwoven Polypropylene Fabrics. II. Functionality of Synthesized Fabrics’ J Appl Polym Sci, 103, 1911–1917. WARD T L, JUNG H Z, HINOJOSA O AND BENERITO R R (1979), ‘Characterization and Use of Radiofrequency Plasma-Activated Natural Polymers’, J Appl Polym Sci, 23, 1987– 2003. WONG K K, TAO X M, YUEN C W M AND YEUNG K W (1999), ‘Low temperature treatment of linen’, Textile Res J, 69(11), 846–855. YASUDA T, GAZICKI M AND YASUDA H (1984), ‘Effects of Glow Discharges on Fibers and Fabrics’, J Appl Polym Sci: Appl Polym Symp, 38, 201–214. ZAHRAN M K (2006), ‘Grafting of Methacrylic Acid and Other Vinyl Monomers onto Cotton Fabric Using Ce(IV) Ion-Cellulose Thiocarbonate Redox System’, J Polym Res, 13, 65– 71. ZUBAIDI AND HIROTSU T (1996), ‘Graft Polymerization of Hydrophilic Monomers onto Textile Fibers Treated by Glow Discharge Plasma’, J Appl Polym Sci, 61, 1579–1584.

5 Modification of textile surfaces using electroless deposition S . Q . J I A N G and R . H . G U O

The Hong Kong Polytechnic University, China

Abstract: Metallisation is a technique with great potential for textiles in terms of both functional and decorative applications. The use of electroless plating to modify textile surfaces is described as a coating treatment in which performance aspects are important from a commercial perspective. The purpose of this chapter is to allow the reader to acquire an understanding of the properties of electroless plating with copper and nickel on polyester fabric. The characteristics of metallised polyester fabrics, including their microstructures, mechanical performance and functional properties, are investigated. Key words: modification, electroless plating, copper, nickel, polyester fabric.

5.1

Introduction

Metallisation is a metal coating process that adds value to and improves the functions of textile materials (Bertuleit, 1991). With the advent of synthetic fibres, metallised fabrics, in particular, have an identity and a quality of brightness that can create unique properties and decorative images (Higging, 1993; Jiang et al., 2006a). Apart from their specific properties, metallised textile surfaces present a unique futuristic appearance as a result of this processing technique. Textile materials modified with different metals have attracted a great deal of attention owing to their potential applications in technology and design fields. To date, the metallisation technologies of sputtering, vacuum deposition, electroplating and electroless plating have been developed (Smith, 1988; Bula et al., 2006; Jiang et al., 2006b). Electroless plating is a popular pre-treatment method used on non-conductive materials to produce precious metallic effects in manufacturing (Dennis and Such, 1993). It is used to generate an armoured surface of solid materials by means of a chemical redox reaction (Gao et al., 2005; Wang et al., 2005; Zou et al., 2006; Rams et al., 2007). Unlike electroplating, where externally supplied electrons act as a reducing agent, in electroless plating, metallic films are formed as a result of a chemical reaction between a reducing agent and metal ions present in the solution (Mallory, 1990; Petukhov, 2007). Electroless plating is an effective process for plating a metallic film on the surface 108

Modification of textile surfaces using electroless deposition

109

of a dielectric textile substrate (Sen, 2001; Shukla et al., 2001; Borges et al., 2003; Liu et al., 2005; Hsu and Huang, 2006; Liu et al., 2006). In the electroless plating process, the metallic ions are reduced to pure metal on a specific type of surface in the presence of catalysts before the beginning of the reaction. The autocatalytic nature of the process indicates that plating continues as long as the substrate is immersed in the plating bath, giving the unique feature of unlimited metal-film thickness. Electroless plating allows the easy formation of continuous and uniform metallic particles on the surface of a substrate in a complex shape, and can be performed on any type of textile – such as fibre, yarn, fabric or clothing – without altering its structure or texture. In previous research, the electroless plating method has been successfully applied to fibrous materials. Both the mechanical and functional properties of electroless-plated textiles have been studied (Marchini, 1991; Gabbay, 1996; Zhang et al., 2001; Jiang et al., 2005; Guo et al., 2007). This chapter introduces the effect of the electroless plating of copper and nickel on polyester fabric. It also discusses the effectiveness of the electroless plating of copper and nickel involving thin metallic films and polyester fabric. In order to characterise the properties of electroless plating, this study evaluates the final performance of electroless copper- and nickel-plated polyester fabrics individually. A basic study is carried out to analyse the modification of polyester fabrics, including their microstructures, mechanical performance and functional properties. This chapter is organised as follows: Section 5.2 provides information on the basic principles of electroless plating. Section 5.3 focuses on the characterisation of copper- and nickel-plated polyester fabrics in terms of their microstructures, mechanical performance and functional properties. Sections 5.4 and 5.5 highlight the strengths and weaknesses of copper- and nickel-plated polyester fabrics and describe the future investigations necessary to facilitate a comprehensive understanding of the electroless plating.

5.2

The techniques and key principles of electroless deposition

To localise the metal film on a particular surface, the substrate should be treated with a catalyst by means of sensitisation with an acid tin (II) chloride solution and activated with an acid palladium chloride solution. In the first step of sensitisation, Sn2+ ions are adsorbed on the non-conducting substrate from a solution of stannous ions. The second step in the process is activation, which is the nucleation effect. Nucleation is performed on the surface of a sensitised substrate by immersing the sample in the activating solution. The surface reaction occurs between the stannous ions, Sn2+, absorbed into the substrate, and the palladium ions, Pd2+, in the nucleated solution. When the sensitised tin on the substrate is placed in the palladium salt solution, all the divalent tin on the surface is eliminated and the

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= Sn ion

Pd

= Pd Pd

5.1 Colloidal activation: palladium cluster enveloped by tin on the substrate surface.

quantity of palladium metal clusters enveloped by tin chloride layers is picked up on the surface of fibres, as shown in Fig. 5.1. The dispersed particles of palladium atoms form the autocatalytic centre on the surface of a substrate. Using a simplified model, the overall nucleation reaction that occurs on the surface of a substrate is shown by: Sn2+ + Pd2+ → Sn4+ + Pd0

[5.1]

The chemical reaction in electroless plating between the metal ions and the reducing agent results in both oxidation and reduction (redox) electromechanical mechanisms, reactions that involve the transfer of electrons between reacting chemical species. The oxidation of a substance is characterised by the loss of electrons, whereas reduction is distinguished by a gain of electrons. Furthermore, oxidation describes an anodic process, whereas reduction indicates a cathodic action. The overall reaction of electroless plating can be explained by (Paunovic, 1998): MZ+ + Red → M + Ox [5.2] where: MZ+ is the metal ion, Red is the reducing agent, M is the metal atom and Ox is the oxidation product of the reducing agent Red. The overall reactions of the electroless plating of copper and nickel are respectively introduced by the equations: Cu2+ + 2HCHO + 4OH– → Cu ↓ + 2HCOO– + 2H2O + H2↑

[5.3]

Ni2+ + 2H2PO2– + 2H2O → Ni ↓ + 2H2PO3– + H2↑ + 2H+

[5.4]

and

In the current authors’ work, a 100% white polyester plain-weave fabric of 0.26 mm in thickness and 76 g/m2 in weight with 38 ends/cm and 36 picks/cm was

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Table 5.1 Composition and condition of electroless plating of copper and nickel Electroless plating

Composition*

Conditions

Copper

10 g/l copper (II) sulphate 5-hydrate 45 g/l potassium sodium tartrate 10 g/l sodium hydroxide 20 ml/l formaldehyde solution

28 ± 1 ºC, pH 11 20 min

Nickel

15 g/l nickel (II) sulphate 7-hydrate 40 ± 1 ºC, pH 9 8 g/l tri-sodium citrate 20 min 18 g/l ammonium chloride A few drops of sodium hydroxide 15 g/l sodium hypophosphite monohydrate

* All chemicals used were analytical reagent (AR) grade.

used. A typical plating solution consisted of: a metal salt; a reducing agent; complexing agents, which enhance the autocatalytic process; buffers; and stabilisers, which retard the reaction in the bulk and promote the autocatalytic process. In order to facilitate the deposition of copper and nickel particles on the surface of polyester fabric, the electroless plating process involved the following six stages. (1) Pre-cleaning. In the pre-treatment stage, all fabric samples were cleaned in a 2% non-ionic detergent at pH 7 and 40 ºC for 20 min to remove any oil or impurities that might have been randomly scattered on the fabric surface during the manufacturing process. The fabric samples were then rinsed with deionised water for about 5 min. (2) Sensitisation. The cleaned fabric samples were subjected to the surface sensitiser, which was a mixture of 5 g/l stannous chloride and 5 ml/l hydrochloric acid (concentration 37%), and were slowly agitated for 10 min at 25 ºC and pH 1. The sensitised fabrics were then rinsed with deionised water. (3) Activation. The fabric samples were immersed in the activator, which was composed of 0.3 g/l palladium (II) chloride, 0.5 ml/l hydrochloric acid (concentration 37%) and 20 g/l boric acid, at pH 2 and 25 ºC for 5 min to achieve surface activation. The activated fabrics were then rinsed with deionised water. (4) Electroless plating. Both bath composition and the operating conditions for the electroless copper and nickel plating of the polyester fabric are shown in Table 5.1. Formaldehyde, a reducing agent, is used in the electroless copper plating process, and tartrate is employed as a complexing agent. Sodium hypophosphite is the reducing agent used for electroless nickel plating. Sodium citrate is used as a buffer and complexing agent. The activated fabrics were immersed in the plating solution for 20 min and were constantly stirred to produce the metallising reaction. All the metallised polyester fabric

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samples were rinsed with deionised water at 40 ºC for 20 min after the metallising treatment. (5) Post-treatment. The cleaned metallised polyester fabrics were then cured by steaming machine at 150 ºC for 1 min. All fabric samples were conditioned under standard atmospheric pressure at 65 ± 2% relative humidity and 21 ± 1 ºC for at least 24 h before evaluation. (6) Evaluation. The metallised polyester fabric samples were observed using a scanning electron microscope (SEM) and X-ray diffraction (XRD) to study the surface morphologies and crystal structures of the deposited copper and nickel films. The mechanical performance of untreated and electroless-plated polyester fabrics was evaluated in terms of weight, thickness, bending rigidity, fabric shrinkage, tensile strength, tearing strength, fabric appearance, wettability and absorbency performance. The functional properties of the metallised polyester fabrics were evaluated, including their conductivity and anti-static properties, electromagnetic interference (EMI) shielding, ultraviolet (UV) radiation protection, and anti-bacterial ability.

5.3

Characterisation of electroless copper- and nickel-plated polyester fabrics

5.3.1

Microstructure

Electroless copper- and nickel-plated polyester fabrics were studied using a field emission scanning electron microscope (JSM-6335F). SEM micrographs of untreated, copper-plated and nickel-plated polyester fibres are shown in Figs 5.2, 5.3 and 5.4, respectively. The polyester fibres were uniformly covered with dense, metallic particles, which are clearly observable in the SEM micrographs. These micrographs show the distributions and covering abilities of the respective metallic particles on the uniform grain fibre surface. The SEM images show the grain growths determined in copper- and nickel-plated polyester fabrics. The average aggregated grain size of copper film was determined to range from 150 nm to 800 nm, and that of the nickel film ranged from 400 nm to 1000 nm. The surface morphologies of the thin films indicate that the deposits effectively changed the polyester fabric surface with a mass particle cluster effect. The crystal structures of the copper and nickel deposits were investigated using X-ray diffraction (XRD, Cu Kα radiation and a graphite filter at 40 kV and 40 mA). Figure 5.5(a) shows the 2θ X-ray diffraction patterns for the copperplated polyester fabric. The strong characteristic peaks of copper appear in the copper-plated fabric. The deposited copper film exhibits the characteristics of a face-centred cubic crystalline structure, which implies that the copper-plated polyester fabric has perfect conductivity. Figure 5.5(b) shows the XRD patterns for the nickel-plated polyester fabric. During the electroless nickel plating process, phosphorous was co-deposited along with the nickel particles. The phosphorus

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113

(b)

5.2 Scanning electron micrographs of original polyester fibres: (a) × 1000; (b) × 30 000.

(a)

(b)

5.3 Scanning electron micrographs of copper-plated polyester fibres: (a) × 1000; (b) × 30 000.

(a)

(b)

5.4 Scanning electron micrographs of nickel-plated polyester fibres: (a) × 1000; (b) × 30 000.

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20

40

60

Cu (311)

Cu (220)

Cu (200)

Intensity (a.u.)

Cu (111)

(a)

80

100

2θ

Intensity (a.u.)

(b)

20

30

40

50

60

70

80

90

100

2θ

5.5 XRD patterns of: (a) copper-plated polyester fabric; (b) nickel-plated polyester fabric.

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115

content controls the microstructure of the as-deposited Ni–P alloy. There is a broad diffraction peak at 2θ = 40º–50º, which indicates that the Ni–P deposit was in an amorphous state; this is consistent with the knowledge that the content is above 7 wt% (Balaraju et al., 2006; Baskaran et al., 2006). The amorphous state of the Ni–P layers is largely attributable to the distortion of the crystalline lattice of nickel caused by phosphorus atoms. It has been shown that most of the properties of electroless nickel are structure-dependent and that the structure is closely related to the phosphorus content of the plating.

5.3.2

Mechanical performance

Weight and thickness Fabric specimens (20 cm × 20 cm) were measured using a weight meter (Shimadzu BX300 meter) both before and after treatment. Based on the reaction time, the average speed of deposition in weight (expressed in g/min) was calculated using: Sw = (Wf – Wo)/t

[5.5]

where: Sw is the average speed of deposition in weight (g/min), Wf is the final weight after treatment (g), Wo is the original weight (g) and t is plating time (min). The thickness of the fabric both before and after treatment was measured using a fabric thickness tester (Hans Baer AG CH-Zurich Telex 57767) at a pressure of 5 g/cm2. The average speed of the thickness increment (expressed in mm/min) was calculated using: St = (Tf – To)/t

[5.6]

where: St is the average speed of thickness increment (mm/min); Tf is the final thickness after treatment (mm), To is the original thickness (mm) and t is plating time (min). The calculated changes in the weight and thickness of the electroless copperand nickel-plated polyester fabrics are shown in Table 5.2. Where the plating reaction took place over 20 min, the percentage increases in weight were 19.7% and 23.7% for the copper- and nickel-plated polyester fabrics, respectively. In terms of thickness, the copper- and nickel-plated polyester fabrics increase by 7.7% and 11.5%, respectively. The weight and thickness increases calculated for the copper- and nickel-plated polyester fabrics differ from each other. The results indicate that copper and nickel particles were deposited on the surface of the polyester fabrics in an effective manner. Where deposition took place over a total period of 20 min, the calculated results reveal that the deposition rates for copper, expressed in terms of weight and thickness, are 0.75 g/min and 1 × 10–3 mm/min, and that the corresponding deposition rates for nickel are 0.9 g/min and 1.5 × 10–3 mm/min, respectively.

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Table 5.2 Changes in weight and thickness of the polyester fabrics after electroless plating Polyester fabric

Weight (g/m2)

Weight increase (%)

Thickness (mm)

Thickness increase (%)

Untreated Copper-plated Nickel-plated

76 91 94

– 19.7 23.7

0.26 0.28 0.29

– 7.7 11.5

Bending rigidity Bending properties have a major influence on both the handling and tailoring performances of fabrics. A FAST-2 bending meter (Ryde, Australia) was employed to measure the degree of bending rigidity of the fabric in both warp and weft directions. The untreated, copper-plated and nickel-plated fabric samples were tested for stiffness in both directions, as directed in the ASTM standard, ASTM D1388–1996. The flexural rigidity of the fabrics was calculated from the fabric bending length together with fabric mass per unit area using: G = W × C3

[5.7]

where: G is the average flexural rigidity (N. cm), W is the fabric mass per unit area (N/cm2) and C is fabric bending length (cm). The bending rigidities of the untreated polyester and metal-plated polyester fabrics are shown in Table 5.3. After electroless plating treatment with copper and nickel, there is an increase in the hardnesses of the copper- and nickel-plated polyester fabrics. The increases in the bending rigidity levels of the copper- and nickel-plated polyester fabrics are estimated to be 12.5% and 6.3% in the warp direction and 21.4% and 7.1% in the weft direction, respectively. The bending rigidity results indicate that the copper- and nickel-plated polyester fabrics are slightly stiffer than the original fabric. Fabric shrinkage The dimensional changes in the metallised fabrics were measured in accordance with AATCC Test Method 135–2003. Table 5.3 shows the calculated percentages of shrinkage in the metal-plated polyester fabrics in both warp and weft directions with respect to Equation 5.8. A slight change in dimensions after five cycles of home laundering is observed. All of the metal-plated polyester fabrics underwent a dimensional change of –0.7% in both warp and weft directions, which is considered insignificant in comparison with the untreated polyester fabric. Based on the measured dimensional results, the metal-plated fabrics are confirmed to undergo a small change in terms of area shrinkage in the course of subsequent

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Table 5.3 Mechanical performance of the untreated, copper-plated and nickelplated polyester fabrics

Polyester fabric

Bending rigidity

Dimensional change

Tensile strength

Bending (µN m)

Shrinkage (%)

Break load (N)

Warp Untreated 1.6 Copper-plated 1.8 Increase (%) +12.5 Nickel-plated 1.7 Increase (%) +6.3

Tearing strength Tearing strength (Nf)

Weft

Warp

Weft

Warp

Weft

Warp

Weft

1.4 1.7 +21.4 1.5 +7.1

0 –0.7 – –0.7 –

0 –0.7 – –0.7 –

211.6 228.1 +7.8 237.5 +12.2

192.6 213.8 +11.0 220.0 +14.2

9.0 7.0 –22.2 6.3 –30.0

8.2 7.1 –13.4 5.3 –35.4

metallising treatment. The same shrinkage values were obtained in both warp and weft directions, which implies that both the copper- and nickel-plated polyester fabrics have a very stable structure. DC = [(B – A)/A] × 100

[5.8]

where: DC is the average dimensional change (%), A is the original dimension (cm) and B is the final dimension after laundering (cm). Tensile strength The tensile properties of the fabric samples were evaluated in accordance with ASTM D 5034–1995 using an Instron 4411 tensile tester (Instron, USA). The tensile strength of a fabric depends on the type of fibre, and the fineness, twist and tenacity of the yarns. It also depends on the weave and yarn density (set). Table 5.3 shows that the tensile strengths of the polyester fabric are enhanced in both warp and weft directions after electroless plating treatment. The breaking load of the copper-plated polyester fabric shows increases of 7.8% and 11% in the warp and weft directions, respectively, compared with the untreated polyester fabric; the breaking load of the nickel-plated polyester fabrics shows corresponding increases of 12.2% and 14.2%. As illustrated in the SEM images, the metallic particles are deposited within the inter-fibre structure. Fibre friction plays an important role in the tensile strength properties of fabrics. The tensile strength results for the polyester fabrics exhibit a higher tolerance to the chemical treatments during the electroless plating processes. Metallising treatment has been proven to result in great improvement in the tensile strength of polyester fabric. Tearing strength Resistance to tear propogation in coated fabrics is very important when such fabrics are under tension. The tearing strengths of untreated, and copper- and

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nickel-plated polyester fabrics were measured with an Elmendorf tearing tester (Thwing-Albert Instrument Co., USA) in accordance with ASTM D 1424–1996, as shown in Table 5.3. The measured results demonstrate that the tearing strength of the fabrics is reduced by the electroless copper and nickel plating process, as reflected by the decreases of 22.2% and 30.0% in the warp direction and 13.4% and 35.4% in the weft direction, respectively. The measured results indicate that interfibre friction restricts the sliding action of the yarns during tearing, thereby reducing the tearing strength values significantly. In addition, the fabric was slightly damaged when exposed to the electroless plating processes with acid (i.e. pH 1 in sensitisation and pH 2 in activation) and alkali (i.e. pH 11 in electroless plating) solutions. Wettability and absorbency In accordance with ASTM D5725–1999, the surface wettability and absorbency of the untreated, and copper- and nickel-plated fabrics were determined using an automated contact angle tester. The fabric was placed horizontally on a glass plate without any creases and the absorption of droplets was observed using a monitor. A 0.1 ml drop of deionised water was placed on the fabric surface and the contact angle was measured using a CAM-Micro (Tantec Inc., Denmark) contact angle meter. The time required for the droplet to absorb completely into the fabric was measured using a stopwatch. The results of the experiment showed that the untreated polyester fabric absorbed water within 10 s, while the copper- and nickel-plated polyester fabrics showed good water repellent properties, with absorption times of more than 30 s. Contact angles of 78º and 98º were observed for the copper- and nickel-plated polyester fabrics, respectively. These results indicate that the hydrophobicity of the fabrics increased following the electroless plating treatment. Fabric appearance After five cycles of standard laundering, the handling performance of the metalplated samples was evaluated in accordance with ASTM D 3691–2002. As the coating materials would have restricted the movement of the yarn structure during laundering, the whole sample showed no significant change in configuration. This property shows that the congeries of metallic particles in the inter-fibre structure might reduce yarn shift under laundering conditions. The resiliency of the metalplated fabrics was better than that of the untreated specimens. The fabric surface handled smoothly after chemical plating. In addition, texture measurements were taken in accordance with AATCC Test Method 124–2006. The copper-plated and nickel-plated polyester fabrics were evaluated for smoothness appearance, with both fabrics producing Grade 4 ratings, implying that they had a smoother appearance than the untreated polyester fabric, which were rated Grade 3.5. These

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119

results confirm that metal plating can enhance the resiliency of fabric and its smoothness.

5.3.3

Functional properties

Conductivity and anti-static property Surface resistance was measured using the four-probe method described in ASTM F 390–1998. The conductivity of both specimens improved after metallising treatment, with resistance being reduced from 1 × 1014 ohm/sq. to 0.7 ohm/sq. for copper-plated polyester and to 1.3 ohm/sq. for nickel-plated polyester. Based on the XRD results shown in Fig. 5.5, the conductivity of nickel deposits with amorphous structure is poorer than copper deposits with a face-centred cubic crystalline structure. Hence, comparing these two results in warp and weft directions, the conductivity of the copper-plated polyester (which increased 7.7% in thickness) was better than that of the nickel-plated polyester (which increased 11.5% in thickness). These results for the conductivity of metallised fabrics explain the significantly reduced static charge on the fabric (Temmerman, 1992). The anti-static properties of the fabric samples were determined by resistance measurement using a Static Voltmeter R-1020 (Rothschild, Switzerland). The average half-life values measured for both the copper-plated and nickel-plated polyesters were less than 0.5 s in both warp and weft directions. The smaller static half-lives of the polyester fabric samples were obtained after the metallising treatment of polyester fabric. The development of an ability to prevent the buildup of static charge on the polyester fabric was confirmed by the good anti-static results observed following electroless plating treatment with copper and nickel. Electromagnetic interference shielding Electromagnetic waves are emitted by a variety of electrical and electronic appliances that influence an integral part of our lives. The electromagnetic waves emitted may interfere with other appliances and also influence people’s health, as well as the environment (Stegmaier et al., 2003). EMI shielding effectiveness (SE) was measured by following ASTM D 4935–1999 using a vector network analyser (Agilent-E8363A, USA) equipped with a synthesised frequency source and a scattering parameter (S-parameter) test set over a frequency range of 2–18 GHz. Based on the results of the incident wave as it undergoes reflection from the surface, absorption by the fibres and secondary reflection, SE is expressed in decibels (dB) using the following equations (Paul, 1992; Kodali, 1996): SEdB = 20 log Ei/Et

(for an electrical field)

[5.10]

SEdB = 20 log Hi/Ht

(for a magnetic field)

[5.11]

120

Surface modification of textiles ü Untreated polyester fabric õ Copper-plated polyester fabric ¸ Nickel-plated polyester fabric

60

50

EMI SE (dB)

40

30

20

10

0 0

2

4

6

8

10

12

14

16

18

20

Frequency (GHz)

5.6 Electromagnetic interference shielding effectiveness of untreated polyester fabric („), copper-plated polyester fabric (z) and nickel-plated polyester fabric (¸).

where: SEdB is the shielding efficiency, Ei is the incident electric wave intensity (V/ m), Et is the transmitted electric wave intensity (V/m), Hi is the incident magnetic wave intensity (A/m) and Ht is the transmitted magnetic wave intensity (A/m). Figure 5.6 shows that the EMI SE results for the polyester fabrics increases after electroless plating using copper and nickel treatments. The SE of the untreated polyester fabric is significantly lower at frequencies of between 2 GHz and 18 GHz. However, the SE results for copper-plated polyester fabric show high values between 45 dB and 55 dB, and the results for nickel-plated polyester fabric range from 35 dB to 40 dB at frequencies between 2 GHz and 18 GHz. These results indicate that electroless copper- and nickel-plated polyester fabrics can attenuate the penetrability of electromagnetic waves. Ultraviolet radiation protection The UV-blocking effects of both the untreated and metallised polyester fabric samples were measured in accordance with the Australia/New Zealand standard AS/NZS 4339–1996 method using a Cary 300 Conc UV–visible spectrophotometer (Varian, USA) with the wavelength scope ranging from 280 nm to 400 nm. The UV protection factor (UPF) values were recorded and the results for -sun-protective clothing were classified according to the UPF rating.

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Table 5.4 Ultraviolet protection factors of the untreated, and copper- and nickelplated polyester fabrics Calculated UPF Polyester fabric

Result

Increase (%)

Untreated Copper-plated Nickel-plated

101.70 229.68 346.52

– 125.84 240.73

During the electroless plating process, the metallic particles are deposited not only on the fibre surface, but also in the spaces between the yarns in the polyester fabric. The coverage of the spaces can prevent UV radiation from penetrating the fabric and reaching the skin. The copper-plated and nickel-plated polyester fabrics indicate the increases in UPF to 229.68 and 346.52, as shown in Table 5.4. These increased UPF results show that the overall ratings of the copper- and nickel-plated polyester fabrics should be graded as 50+, demonstrating that the UV radiation protection of the copper-plated and nickel-plated polyester fabrics is excellent. When compared with the untreated polyester fabric, the mean UPF for the two metallised polyester fabrics increase by 125.84% and 240.73%, respectively. These results indicate that metallised polyester fabrics are capable of providing excellent protection from UV radiation. Anti-bacterial ability Various finishing processes have been developed for bacterial and pathogenic fungi resistance in textile materials. In previous research, silver, which is known as a disinfectant, has received attention in the medical field, where the proximity of metal to body tissues appears to have antibiotic properties, assists in the healing of deep wounds and broken bones, and provides conductivity for electrical therapy (Ford, 1988). In this experiment, the anti-bacterial properties of the electroless copper- and nickel-plated polyester fabrics were quantitatively evaluated with Staphylococcus aureus (American Type Culture Collection (ATCC) 6538) and Escherichia coli (ATCC 11229) in accordance with the AATCC Test Method 100–2004. The percentage bacterial reductions realised by the anti-bacterial function of the treated fabrics were calculated according to the following equation: R = [(B – A)/B] × 100%

[5.12]

where: R is the percentage bacterial reduction, B is the number of bacterial colonies at the initial stage (0 h) and A is the number of bacterial colonies after 24 h of contact with the finished substrates.

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The results show that, for S. aureus, the bacteria reduction rates for the copperand nickel-plated polyester fabrics reached 99.6% and over 99.9%, respectively; for E. coli, these bacteria reduction rates were reached over 99.9% and 94.4%, respectively. In the experiment, the copper-plated polyester fabric had a particularly high anti-bacterial effect against the two kinds of bacteria; in the nickel-plated polyester, anti-bacterial activity against S. aureus was better than against E. coli. The anti-bacterial mechanism of the metal is a result of the effect of the metallic particles on the external membrane of the bacteria; i.e. the metallic particles block their respiratory capacities, thereby preventing them from breeding.

5.4

Strengths and weaknesses of electroless deposition

The properties of electroless plating of copper and nickel on polyester fabrics have been discussed with respect to SEM and XRD analysis, fabric weight, thickness, bending rigidity, shrinkage, tensile strength, tearing strength and water-repelling performance. Electroless plating appears to be an effective method for modifying the surface of polyester fabric, and has extensive potential for textile products in terms of both functional and decorative applications. In particular, copper- and nickel-plated polyester fabrics showed significant improvements in functional performance, including conductivity, EMI shielding, UV protection and antibacterial properties. Based on the experimental results, electroless plating with copper and nickel has the following advantages: (a) non-conducting polyester fabric can be plated; (b) the plating process is simple and does not require electrical energy; (c) copper and nickel particles are uniformly distributed on both sides of the fabric without significantly altering other properties such as density, flexibility and handle of the fabric; (d) specific functional properties of copper- and nickelplated polyester fabrics have been revealed. The realisation of electroless plating on polyester fabric has opened up a new field in textile materials. Metallised textiles can improve protective performance in terms of quality and technical aspects of the fabric. The copper- and nickel-plated polyester fabrics also meet the aesthetic requirements and fulfil wear-comfort for the respective uses. However, each combination of metal and reducing agent in electroless deposition requires a specific pH range and bath formulation. The tearing force of polyester fabrics has shown deterioration in both warp and weft directions when the fabric has been subjected to electroless plating. This is probably due to the fact that the high and low pH values during plating and pretreatment will damage fibre interbondings, resulting in weakening of the fibres. In addition, electroless plating solutions are especially susceptible to impurities that affect the processing solution. Impurities in the solution can cause reduced ductility as well as pitting, adhesion and roughness. The metal particles only have a moderate adhesive strength on the surface of fabrics; therefore, metal-plated

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fabrics have a weakness in that when the plated metal is exposed to air it may peel off. The actual production of copper- and nickel-plated fabrics using the electroless plating method is a multistage process that requires complex chemicals. After each stage, the fabrics are carefully washed, thus yielding a large amount of effluent that requires intensive wastewater treatment.

5.5

Future trends

Electroless plating is a finishing process that can bring out unique properties of textiles. Retailers and consumers are increasingly demanding specific functions, added value (such as appearance) and improved comfort in textiles. Electroless plating is a unique process that has the potential to provide functional materials with a metallic effect and good handling characteristics. Increasing levels of EMI will prompt an accompanying rise in the need for shielding. An increasing demand for electrically conductive textiles is emerging in the market. There is also growing demand for materials that can protect individuals from electromagnetic radiation. The future development of metal-plated fabrics will be based on their remarkable shielding properties and will result in wide applications. Pre-treatment by micro-roughing, e.g. by etching and plasma treatments, can be used on the fibre surface before metal deposition. This will help to enhance the mechanical bond between metal particles and fibres. Appropriate coating materials can be applied onto fibre surfaces as a post-treatment process to improve the wear resistance. The coated fabrics may maintain the stability of metal performance for a longer time when exposed to a complex climate. For instance, the oxygenation-prevention properties can be enhanced by a finishing treatment. Further investigation is required to develop precise plating conditions and finishing processes to achieve particular properties. Metal-plated fabrics will continue to serve specialised requirements for heat provision, heat protection, static control, and a variety of high technology needs associated with radiation. In addition, the development of electroless-plated fabric designs is a new area of application that will bring benefits in the area of textile product decoration.

5.6

Sources of further information and advice

GUO R H, JIANG S Q, YUEN C W M AND NG M C F (2009), ‘An alternative process for electroless copper plating on polyester fabric’, J Mater Sci: Mater Electron, 20(1), 33–38. JIANG S Q, KAN C W, YUEN C W M AND WONG W K (2008), ‘Electroless nickel plating of polyester fiber’, J Appl Polym Sci, 108, 2630–2637.

5.7

References

BALARAJU J N, NARAYANAN T S S AND SESHADRI S K (2006), ‘Structure and phase transformation behaviour of electroless Ni–P composite coatings’, Mater Res Bull, 41(4), 847–860.

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BASKARAN I, SANKARA T S N AND STEPHEN N A (2006), ‘Effect of accelerators and stabilizers on the formation and characteristics of electroless Ni–P deposits’, Mater Chem Phys, 99(1), 117–126. BERTULEIT K (1991), ‘Silver coated polyamide a conductive fabric’, J Coated Fabrics, 20, 211–215. BORGES N G, AVILA A O V AND SOLIDI V (2003), ‘Electroless plating of nickel-phosphorous on surface-modified poly(ethylene terelphthalate) films’, Appl Surf Sci, 220, 238–250. BULA K, KOPROWSKA J AND JANUKIEWICZ J (2006), ‘Application of cathode sputtering for obtaining ultra-thin metallic coatings on textile products’, Fibers & Text in Eastern Eur, 14(5), 75–79. DENNIS J K AND SUCH T E (1993), Nickel and Chromium Plating, 3rd edn, Cambridge, Woodhead Publishing Ltd. FORD J E (1998), ‘Metallized textiles’, Textiles, 17(3), 58–62. GABBAY J (1996), Applications of metallized textiles, U.S. patent, 5,981,066. GAO J N, TANG F Q AND REN J (2005), ‘Electroless nickel deposition on amino-functionalized silica spheres’, Surf Coat Technol, 200, 2249–2252. GUO R H, JIANG S Q, YUEN C W M AND NG M C F (2007), ‘Surface characterisation of electroless silver-plated polyester fibres’, Res J Textile Apparel, 11(3), 48–53. HIGGING J P P (1993), Cloth of Gold: A History of Metallized Textiles, London, The Lurex Company Limited. HSU J M AND HUANG B R (2006), ‘The growth of silicon nanowires by electroless plating technique of Ni catalysts on silicon substrate’, Thin Solid Films, 514, 20–24. JIANG S Q, NEWTON E, YUEN C W M AND KAN C W (2005), ‘Chemical silver plating and its application to textile fabric design’, J Appl Polym Sci, 96, 919–926. JIANG S Q, YUEN C W M, KAN C W AND SUN S (2006a), ‘Study of metallisation on textile fabrics’, in Proceedings of the 35th Textile Research Symposium, Hangzhou, People’s Republic of China, pp. 125–129. JIANG S Q, NEWTON E, YUEN C W M AND KAN C W (2006b), ‘Chemical silver plating on polyester/cotton blended fabric’, J Appl Polym Sci, 100, 4383–4387. KODALI V P (1996), Engineering Electromagnetic Compatibility: Principles, Measurements, Technologies, New York, Institute of Electrical and Electronics Engineers, Inc. LIU C M, LIU W L, HSIEH S H, TSAI T K AND CHEN W J (2005), ‘Interfacial reaction of electroless nickel thin films on silicon’, Appl Surf Sci, 243, 259–264. LIU W L, HSIEH S H, TSAI T K, CHEN W J AND WU S S (2006), ‘Temperature and pH dependence of the electroless Ni-P deposition on silicon’, Thin Solid Films, 510, 102– 106. MALLORY G O (1990), ‘The fundamental aspects of electroless nickel plating’, in Mallory G O and Hajdu J B (Eds), Electroless Plating: Fundamentals and Applications, Orlando, Florida, The American Electroplaters and Surface Finishers Society, pp. 1–56. MARCHINI F (1991), ‘Advanced applications of metallized fibres for electrostatic discharge and radiation shielding’, J Coated Fabrics, 20, 153–165. PAUL C R (1992), Introduction to Electromagnetic Compatibility, New York, John Wiley & Sons, Inc. PAUNOVIC M (1998), Fundamentals of Electrochemical Deposition, New York, John Wiley & Sons, Inc. PETUKHOV I V (2007), ‘Of the mechanism governing the growth of electrolessly deposited nickel-phosphorus coatings’, Russian J Electrochem, 43(1), 34–41. RAMS J, UREN A, ESCALERA M D AND SÁNCHEZ M (2007), ‘Electroless nickel coated short carbon fibres in aluminium matrix composites’, Composites: Part A, 38, 566–575.

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SEN A K (2001), Coated Textiles: Principles and Applications, Pennsylvania, Technomic Publishing Company, Inc. SHUKLA S, SEAL S, AKESSON J, ODER R, CARTER R AND RAHMAN Z (2001), ‘Study of mechanism of electroless copper coating of fly-ash cenosphere particles’, Appl Surf Sci, 181, 35–50. SMITH W C (1988), ‘Metallized fabrics – techniques and applications’, J Coated Fabrics, 17, 243–253. STEGMAIER T, SCHMEER L G AND PLANCK H (2003), ‘Textiles with electromagnetic protective effect’, Text Technol Overseas, 4, 35–37. TEMMERMAN L D (1992), ‘New metallized materials for EMI/RFI shielding’, J Coated Fabrics, 21, 191–199. WANG F, ARAI S AND ENDO M (2005), ‘The preparation of multi-walled carbon nanotubes with Ni–P coating by an electroless deposition process’, Carbon, 43, 1716–1721. ZHANG M C, KANG E T, NEOH K G, AND TAN K L (2001), ‘Electroless plating of copper and nickel on surface-modified poly(tetrafluoroethylene) films’, J Electrochem Soc, 148(2), C71–C80. ZOU G Z, CAO M S, LIN H B, JIN H B, KANG Y Q AND CHEN Y J (2006), ‘Nickel layer deposition on SiC nanoparticles by simple electroless plating and its dielectric behaviors’, Powder Technol, 168, 84–88.

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SEN A K (2001), Coated Textiles: Principles and Applications, Pennsylvania, Technomic Publishing Company, Inc. SHUKLA S, SEAL S, AKESSON J, ODER R, CARTER R AND RAHMAN Z (2001), ‘Study of mechanism of electroless copper coating of fly-ash cenosphere particles’, Appl Surf Sci, 181, 35–50. SMITH W C (1988), ‘Metallized fabrics – techniques and applications’, J Coated Fabrics, 17, 243–253. STEGMAIER T, SCHMEER L G AND PLANCK H (2003), ‘Textiles with electromagnetic protective effect’, Text Technol Overseas, 4, 35–37. TEMMERMAN L D (1992), ‘New metallized materials for EMI/RFI shielding’, J Coated Fabrics, 21, 191–199. WANG F, ARAI S AND ENDO M (2005), ‘The preparation of multi-walled carbon nanotubes with Ni–P coating by an electroless deposition process’, Carbon, 43, 1716–1721. ZHANG M C, KANG E T, NEOH K G, AND TAN K L (2001), ‘Electroless plating of copper and nickel on surface-modified poly(tetrafluoroethylene) films’, J Electrochem Soc, 148(2), C71–C80. ZOU G Z, CAO M S, LIN H B, JIN H B, KANG Y Q AND CHEN Y J (2006), ‘Nickel layer deposition on SiC nanoparticles by simple electroless plating and its dielectric behaviors’, Powder Technol, 168, 84–88.

6 Textile surface functionalisation by chemical vapour deposition (CVD) J. I. B. WILSON

Heriot-Watt University, UK

Abstract: Chemical vapour deposition (CVD) is the process of synthesising a material, often as a coating on another material, from vapour precursors that are caused to react by heating. The basic technique is explained here together with the features that are either advantageous or difficult for coating textiles. Some specific examples are described for both low-temperature and hightemperature materials, where the coating imparts additional functionality such as superhydrophobicity or biocidal action. The future impact of CVD may lie in the demand for more sophisticated products incorporating nanoparticles, for instance, to make intelligent textiles with integrated sensors and actuators. Key words: chemical vapour deposition, precursor chemicals, thin film coatings, textiles.

6.1

Introduction

Treating a surface by means of a vapour-phase chemical reaction enables a conformal coating to be produced that can endow the material with additional functions, from enhanced mechanical or chemical resistance to electrical conductivity or biochemical activity. The basic process of chemical vapour deposition (CVD) is described here as well as the parameters that should be controlled to obtain the desired result, and its applicability to textiles is demonstrated by some real examples. Technical textiles can deliver a wide range of specialist properties, often dependent on the surface composition and structure as well as on the fabric’s texture and shape. The surface in particular is amenable to alteration by both physical and chemical processes, as this book makes obvious, and it may be difficult to identify the best process for a desired modification, because manufacturers do not yet have a history of experience with all of these methods. CVD, the production of a solid coating from a volatile precursor by chemical reaction, has been the basis of several industries, most notably semiconductor device fabrication. Integrated circuits and other semiconductor devices rely on a combination of crystalline semiconductors and amorphous or polycrystalline dielectrics along with metal contacts. Some of these layers are produced by CVD, 126

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but commonly at temperatures of several hundred degrees Celsius. Another industry was a forerunner of the UK chemical company, ICI: in the 1880s, Ludwig Mond and his assistant, Carl Langer, discovered the volatile (and toxic) compound nickel carbonyl (Ni(CO)4), by a low-temperature reaction between CO and Ni. This reaction could be used to extract Ni from its ore and when reversed will plate Ni onto adjacent surfaces, at temperatures around 200 ºC. Thus CVD is usually associated with high temperatures and engineering components, from its early history in metal plating through to more recent developments in ceramics, but it is possible to coat or treat surfaces at temperatures that are acceptable for some textiles and polymer fibres. An alternative chemical deposition scheme retains the useful aspects of CVD but drives the reactions by electrical discharge (plasma-enhanced CVD, PECVD) which enables substrate temperatures to be reduced; even lower temperatures may be possible if the reactions can be driven by photons but these should be shortwavelength ultraviolet (UV) to break chemical bonds, and so can attack many textile substrate materials as well. However although many ‘low-temperature’ thermal CVD processes are still out of reach of commodity textiles some colder reactions have been found that may be applied at temperatures sustainable by textiles. There is a risk that these restricted temperatures may produce incomplete chemical reactions, for instance giving poor adhesion of the product or inclusion of unreacted species in the coating. This difficulty in using low temperatures for CVD can be ameliorated by considering the two factors that control the products from CVD reactions, namely thermodynamics and kinetics, from which it may be possible to design a strategy to redress any problems. In brief, the surface temperature tends to control the growth rate at low temperatures but beyond this point it is the rate of transport of reactants to the surface that determines the growth rate; until at the highest temperatures there are additional homogeneous gas-phase reactions to consider. Most practical CVD reactors operate under non-equilibrium conditions and so chemical kinetics determines the products. An effective CVD reactor must provide controlled growth at a constant rate, preferably without too much sensitivity to changes in the operating conditions. A simple everyday example of undesirable coating by thermally driven CVD is the generation of carbonaceous deposits by gas-fuelled boilers when the supply of air (oxygen) or fuel (e.g. methane) is incorrectly balanced so as to produce soot instead of heat and volatile byproducts. When a similar process, using hydrogen and methane below atmospheric pressure, is driven by electrical discharge (plasma) it is possible to vary the deposit from graphitic carbon to diamond according to the reactant mixture and the temperature. Merely following a recipe may produce a satisfactory result, but when the coating quality drifts in some manner it is usually necessary to seek a deeper understanding of the process instead of using an empirical approach. Given these constraints, what can be achieved by CVD on a textile surface?

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(a)

(b)

(c)

6.1 Scanning electron micrographs of woven polyester fabric: (a) fibres coated with a thin film of vacuum-evaporated aluminium; (b) and (c) fibres coated with nanocrystalline silicon by PECVD from silane and hydrogen at 180 ºC, over an aluminium film.

Typical requirements include hydrophilicity or hydrophobicity, electrical conductance, flame retardance, antistatic or antibacterial action, and a host of other more complex functions. Some of these will be discussed in further detail but not all may yet be conferred by CVD treatments despite the attractions of a process that avoids the need to dispose of liquid waste. Perhaps the most recent technologies are those utilising nanoparticles to add advanced performance to more traditional fabrics (e.g. silver nanoparticles incorporated into medical materials to kill MRSA (methicillin-resistant Staphylococcus aureus)) and these may require chemical attachment for permanency. Nanocrystalline silicon thin films deposited by PECVD onto textile substrates are being explored at Heriot-Watt University for flexible solar cells: Fig. 6.1 shows micrographs of nanocrystalline silicon on top of an aluminium conducting layer, which form the first two layers of the complete device. Some of the other layers may in future be grown by thermal CVD.

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Gas flow Gas phase reactions

Desorbed species Boundary layer

Textile substrate

6.2 Schematic diagram of processes occurring during CVD on a textile. Gas-phase reactions driven by heat produce reactant species that diffuse to the surface, migrate over the surface and either stick or are desorbed again. Any volatile products are also desorbed from the surface. The stagnant boundary layer exists at higher pressures and restricts the flow of species to and from the surface.

6.2

Practical methods for chemical vapour deposition

The essence of CVD is the use of a chemical reaction to form a coating from a vapour, with the reaction byproducts leaving as volatile species (Fig. 6.2). Some volatile precursors are readily decomposed to provide the desired product but at the lowest temperatures they may not fully react as required, thus producing a mixture of essential and unnecessary molecular species. On the other hand, the temperature can be held at a low value without compromising the reaction’s completion and rate, if energy is supplied by electrons, photons or other energetic species, in such processes as PECVD and photo-CVD. In this chapter we shall concentrate solely on thermal CVD, but even within this restriction there are options for using hotwall or cold-wall furnaces, or hot-wire heaters. Apart from the minimum temperature required for the gaseous reaction to proceed, it may be necessary to heat the substrate to prevent the coating from being porous and unconsolidated: this enables the arriving species to migrate across the surface rather than to stick in three-dimensional clusters where they arrive. The wrong conditions (e.g. too high

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a reactant concentration or too cold a substrate) can lead to gas-phase reactions generating a fine powder but unless this is consolidated on the substrate the coating will not be functional even though it is the intended chemical product. The only effective way of reducing the process temperature may be to design new volatile precursors. Another reason for seeking alternative precursors is to avoid the use of toxic or flammable chemicals, which has been a problem with CVD of semiconducting materials: silicon films usually depend on silane chemistry (SiH4 is spontaneously flammable in air) and are doped with small amounts of diborane or phosphine (both extremely toxic); III–V compound semiconductors, such as those using Ga or In with As, P or N, in various combinations, have relied on metal-organic compounds (such as alkyls, which are spontaneously flammable liquids) together with toxic gases (AsH3 and NH3). In each case the vapour or gas is usually supplied to the reactor through an electronic mass flow controller, which monitors the supply using the thermal properties of the gas and uses a feedback loop to maintain it at a preset rate; liquids may be vaporised first by heating, or may be carried by an inert gas that is bubbled through the container at a constant rate. For the higher flow rates demanded by a few techniques, the reactant may be supplied as a mist from a nebuliser. Another decision must be made about the reaction chamber pressure: many practical examples are operated at atmospheric pressure, but others are more effective at low pressure, which requires vacuum pumps, control valves and gauges to maintain a constant pressure as well as measures to handle the exhaust gases safely. (PECVD can be used at atmospheric pressure for a few coating materials but is more commonly operated at pressures below 100 Pa.) One advantage of low-pressure CVD is the thinner boundary layer above the substrate compared with that in atmospheric pressure systems. This layer is a stagnant sheath of gas through which reactants (and byproducts from surface reactions) must diffuse, and consequently deposition at low pressures can be much higher than at high pressures because of faster diffusion times, despite the lower density of reactant. Uniformity of coating is also enhanced at low pressures. Given this wide choice of processing conditions, how is the user to make their selection? Consideration of the construction of textiles and of the scale of the material to be treated may help in this respect; chemical modification of materials should avoid using corrosive and toxic chemicals if possible and should affect only the surface and not the bulk properties. Highly energetic sources such as UV photons or electrons may damage some polymeric materials and can penetrate deep into the polymer causing molecular cross-linking rather than functionalisation. On the other hand, some limited ‘damage’ may be needed to enable fibres to be grafted with other molecules, perhaps in a two-step process involving activation and then attachment. Consideration must be given to disposal of the effluent – usually by gas scrubbers, absorbers or combustion, all of which are preferable to the large volumes of waste that are produced by liquid-based coating treatments.

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The design of a CVD reactor should consider more than just chemical kinetics and chemical thermodynamics if the deposition rate is to be modelled and understood. In turn this requires consideration of heat flow and mass flow within the gas and onto the substrate surface, depending on the temperature gradients within the reactor, including coupled effects such as convection. In this way, the importance of the reactor geometry (e.g. gas feed, heater distribution) becomes apparent for delivering coatings that are uniform across the substrate load on a commercially realistic scale. Now let us return to the question of the dominant controlling parameter in thermal CVD. There are two extremes: either the coating rate is limited by the reaction rate in the gas phase or on the surface of the substrate (following an Arrhenius dependence on temperature) or it is limited by the supply of reactants (‘mass-transfer’ limited). At the lower temperatures preferred for CVD on textiles, it may be necessary to run a surface-reaction-limited process, and this will require close temperature control which is conventionally provided by a hot-wall reactor. One disadvantage is that the walls of the reactor will also become coated and frequent cleaning is necessary. This is even more of a problem when the products of the reaction can spall off the chamber walls. In contrast, if the process is run at relatively high temperatures then its rate will depend on the supply of reactant to the textile surface and a cold-wall reactor may be used because the process is insensitive to small variations in the temperature. This also avoids homogeneous nucleation in the gas itself. However, the configuration of the reactant supply and its detailed flow pattern across the textile will then affect the uniformity of the coating and it is more difficult to design a reliable process. By running the reaction below atmospheric pressure, there is less likelihood of a thick boundary layer forming above the textile surface, through which the reactants must diffuse. In cold-wall reactors heating is provided either by radiant lamps or by radio frequency induction to a conducting susceptor (e.g. graphite) within the reactor. In both situations, some form of process monitoring is desirable to track and correct any changes from the desired conditions. For a flow-through, or roll-toroll, process an in-line measurement of coating properties (e.g. thickness or conductivity) may be sufficiently fast to enable feedback to the process conditions. In other processes, in situ monitoring may be needed and in order to avoid interfering with the reaction this may be restricted to optical techniques (e.g. reflectance). More advanced instrumentation is useful for process development but may be too expensive or specialist for production lines (e.g. laser spectroscopies), or may only operate under special conditions (e.g. surface analysis and mass spectroscopy that usually require vacuum). Many forms of CVD reactor have been developed for the semiconductor industry in which substrates are wafers with a relatively small area that may be processed in batches within a heated reactor tube. Substrates may also be passed close to a heated wire that provides a much more localised reaction. This industry also uses continuous deposition of silicon oxide from silane combustion at

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atmospheric pressure onto wafers passed through by conveyor belt. In atmospheric pressure systems there are obviously complicated heat and gas flows that should be controlled for the production of uniform coatings, taking into account the convection effects that arise from gradients of temperature or gas concentration. CVD reactor design for the semiconductor industry has made extensive use of gas dynamics modelling and experimental observations of flow patterns in particular reactors over a range of pressures. This analysis uses dimensionless parameters that typically describe the ratio of two or more contrasting effects, such as heat flow by convection and diffusion (the Peclet number), or gas buoyancy and viscosity (the Grashof number). In this manner, it may be possible to scale up the design of a reactor from research scale to production scale by altering the values of such controlling parameters as pressure and flow in the correct ratio.

6.3

Characteristics of chemical vapour deposition coatings

In contrast to some vapour coating methods, CVD operates in conditions that give a small mean free path for the species, even when operated below atmospheric pressure, and the consequent scattering of the species reaching the substrate helps to coat all surfaces and not only those in line-of-sight of the vapour entry points. The resulting conformal coverage of stepped or rough surfaces is enhanced if possible by heating the substrate to increase the surface mobility of arriving species (Fig. 6.3). In the semiconductor industry, silicon oxide coatings are produced by several CVD reactions, and the form that is produced when phosphorus is included in the source vapour will soften and flow while still solid, giving a smooth surface over uneven topography. Unfortunately for everyday textiles, this requires temperatures of hundreds of degrees. Alteration of surface texture in this manner is often desirable, but for some reactions a more extreme modification may result from a reversal of deposition towards etching conditions. Again an example may be taken from the semiconductor industry, which at times uses chlorinated silanes as vapour sources of silicon in preference to pyrophoric silane itself. Tetra-chlorosilane (silicon tetrachloride) will decompose to silicon solid and gaseous HCl when heated with hydrogen, but the reaction is reversible and will go in the opposite direction if the concentration of SiCl4 is too high, causing solid silicon to be removed as SiCl2 vapour. It is therefore important to regulate the vapour mixture to ensure a rapid coating process without too much competition from etching, although the balance of the two will have an effect on the morphology of the coating. Another characteristic of thin film coatings is the possibility of built-in stress, generated by the differing thermal expansion coefficients of coating and substrate, or by distorted atomic lattices when defects are included in the structure or the composition is non-stoichiometric. This is obviously at its worst when coatings are

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6.3 Coating from a vapour over a profiled substrate showing (a) conformal layer when species arriving on the surface are able to migrate easily, and the profiles resulting from the absence of surface migration together with a long vapour mean free path (b) and a short mean free path (c). After Adams (1983).

thick and rapidly deposited, and substrates are unheated. For instance, silicon oxide coatings may not have the actual composition given by its usual formula of SiO2 and may include significant amounts of hydrogen, when synthesised by lowtemperature CVD or PECVD. The magnitude of the stress and its sign (compressive or tensile) depend on substrate temperature, growth rate and porosity, among other parameters. We should note that although the requirements for semiconductor device manufacture appear to be remote from the textile industry, high-performance woven and nonwoven fabrics, as well as tapes and braids, are widely available, made of mixtures of silica and other oxide fibres that will withstand high temperatures and severe chemical environments. Other oxides, such as TiO2, may be added by low-temperature techniques to more conventional textiles to impart properties such as self-cleaning (using catalytic action).

6.4

Applications

Some diverse examples of CVD coatings have been chosen to demonstrate the enhanced performance or added functionality that such treatments may bestow on textiles and fibres. Polypyrrole (PPy) is an interesting example of a polymer coating that may confer electrical conductivity on an insulating material and while not as

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conducting as metal films (e.g. ~2 ohm cm compared with ~2 µohm cm), it is much more flexible. Polymerisation of pyrrole is achieved by an oxidative reaction and dopants such as iodine may be incorporated at the same time. Various solution chemical routes may be used, followed by substrate coating by such processes as spin-coating; alternatively, electro-polymerisation may be used to coat a conducting substrate directly (e.g. over-coating a layer of carbon nanotubes). Of more relevance to this chapter is the option of vapour deposition from the monomer onto a surface pre-treated with ferric chloride solution and perhaps a dopant molecule as well (Leung et al., 2006). This route produces very smooth coatings that conform to the fibres in a fabric and offers a sensor with high sensitivity to changes in strain that are detectable by monitoring the coating resistance. Earlier research showed that PPy films may also be prepared by mixing iodine (both acting as oxidant and dopant) and pyrrole vapours at low pressure (3 Torr) and close to room temperature (Asano et al., 1988), giving a growth rate of ~0.1–0.4 µm min–1 for a coating with a conductivity of 0.1 S cm–1. The mechanical properties of PPy do depend on the preparative method, perhaps because polymerisation may occur before adsorption onto the substrate surface, in much the same way as gas-phase homogeneous CVD tends to form dusty deposits with poor adherence compared with heterogeneous surface CVD. In general, dusty deposits are difficult to consolidate and may be more sensitive to atmospheric attack than denser deposits. At the opposite end of the deposition temperature scale is the coating of SiC-type fibres for reinforced ceramic composites. Commercially produced Hi-Nicalon silicon carbide fibre is a mixture of β-silicon carbide crystallites and carbon that may be woven into three-dimensional shapes and reinforced by chemical vapour infiltration to form strong high-temperature composites. The woven matrix may be infiltrated by butane to provide a 300 nm carbon interfacial layer and then the whole may be densified by low-pressure infiltration of trichloromethylsilane vapour carried in hydrogen and argon at 1100 ºC to form silicon carbide. The densification process takes many hours for a typical preform. (Xu et al., 2001). Fracture strength in service of these composites depends on the quality of fibre– matrix bonding and thermal expansion mismatch between the components: it is desirable to eliminate voids between fibre bundles by effective infiltration. The pyrolytic carbon bonding layer mentioned above is one of several means to enhance adhesion within fibre-reinforced composites. Surface modification of polymeric fibres to promote adhesion in a matrix often uses plasma processing (Luo and Van Ooij, 2002) in a similar manner to altering the hydrophobic or hydrophilic nature of polymer surfaces by exposure to plasma. The production of superhydrophobic surfaces that will be self-cleaning but remain transparent has been achieved without plasma by thermal CVD of a polymethylsiloxane coating on cotton cellulose. Superhydrophobicity (water contact angle greater than 150º) depends on reducing the surface energy, as for ‘normal’ hydrophobic surfaces, but with the addition of rough texture on a nanoscale to trap air and prevent wetting. The procedure reported by Li et al. (2007) exposes clean cotton fabric to

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trichloromethylsilane at 50 ºC to chemisorb it onto the fibre surfaces; it is then reacted with pyridine solution at room temperature to hydrolyse any remaining Si–Cl bonds, and is then washed, and finally dried at 150 ºC for 10 minutes. This final step polymerises Si–OH to form a nanoscale silicone coating. The excellent performance of the treated fabric was demonstrated by the very high water droplet contact angle of up to 157º, and its stability after 20 laundry washing cycles. Microscopy revealed that the polymer coating was ~200 nm in thickness and covered the whole surface of the individual fibres without altering their appearance. Superhydrophobic fabrics have also been made in an interesting development of electrospun fibres that combines the hydrophobicity of some nonwoven textiles with the high water repellence of fluorinated polymer coatings (Ma et al., 2005). In general, the extreme values of the liquid contact angle on superhydrophobic surfaces depend partly on a low surface energy (such as that provided by fluorine terminations) and partly on a fine surface roughness, which traps air and prevents droplets from contacting the actual surface. The work of Ma et al. showed that electrospun fibres of polycaprolactone (PCL), a biodegradable but nonhydrophobic polymer, are actually quite hydrophobic at first, if they are sufficiently fine, but that this is not a stable condition because the trapped air film can be displaced, leading to surface wetting. Ma and co-workers stabilised the performance by adding a thin layer of poly-perfluoroalkyl ethyl methacrylate (PPFEMA) by low-temperature, low pressure ‘initiated CVD’ (i-CVD). This modification of low-pressure thermal CVD uses heat to decompose an initiator (in this case tert-butyl peroxide vapour at 250 ºC), forming free radicals that polymerise the monomer vapour for deposition onto a cold substrate. In this way, films of ~70 nm were deposited conformally onto the electrospun fibres. The highest water contact angle was 175º for fibres that were ~100 nm in diameter and with a beaded morphology, compared with 119º for PPFEMA on a flat Si wafer. (The best material was also highly oleophobic.) More ambitious chemical or biochemical functionality is designed to be added by coating fibres with a variety of compounds (for instance, polymeric quaternary ammonium is known to be biocidal). An i-CVD process has been effective in coating nylon fabric with an antimicrobial layer of a styrene-derived polymer (Martin et al., 2007). This approach is of interest for water filtration. Successful use requires a low leaching out of the active agent and easy regeneration when necessary. However, much is still unknown about the biocidal action of these coatings and the optimum attachment of compounds to polymer surfaces. A further modification of conventional thermal CVD known as atomic layer deposition (ALD) was developed for the precise formation of some semiconductor compounds, and it provides a means of adding very thin inorganic coatings to textiles. The procedure involves exposing a substrate to a vapour that contains one of the elements of interest, under conditions that only allow a coating that is one atom thick to form. The vapour is then removed from the process chamber, which

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is purged, and a second vapour is admitted, to react with the first atomic layer, forming a second coat that is also one atom thick. This two-step process is repeated, perhaps several hundred times, until the required thickness of coating has been reached. Under the appropriate conditions of pressure and temperature, a suitable substrate may be coated very uniformly by an accurate thickness of material. The more complicated surface configuration of textiles offers a greater challenge than the flat semiconductor wafers used for electronic and optical devices, but the concept has been proven by researchers at North Carolina State University (Hyde et al., 2007) who have deposited aluminium oxide films on cotton at 100 ºC from sequential 1 or 2 second exposures to low-pressure trimethyl aluminium (TMA) in argon and water vapour: TMA can only bond to the cotton surface at a limited number of sites, thus this step is self-limiting. The subsequent reaction with water forms an oxide layer, with the carbonaceous byproducts removed as volatiles. Coatings other than aluminium oxide (and including metallic coatings) would offer a variety of desirable features for a cotton fabric – such as improved mechanical or electrical properties, fire resistance or UV-catalytic surface chemistry for self-cleaning – while retaining the basic structure of this common fabric. Another method for depositing inorganic coatings that may have a use for textile surface modification uses the room temperature reaction between silane and hydrogen peroxide to synthesise silica. An elegant demonstration of this has shown that delicate biological structures may be replicated by such silica coatings (Cook et al., 2003). Growth rates are reported to be 50–200 nm min–1. The addition of other hydrides (e.g. diborane, phosphine, germane) provides some flexibility in the composition of the inorganic coating, and methylsilane offers a safer alternative to highly pyrophoric silane.

6.5

Current trends and potential advances in uses and techniques

In the search for new functionality on textiles it is apparent that more and more sophisticated coating techniques are being explored, often from other industrial sectors. The restrictions regarding the temperatures that can be used with commodity textiles make it very difficult to transfer some technologies, and equally important is the constant requirement for low-cost techniques. Thus the introduction of nanotechnology may offer some remarkable visions for novel applications involving smart textiles, but unless the methods may be scaled up to full-size production from what are often small scale laboratory procedures, they may fail to meet economic tests. As discussed in Chapter 8 in this volume, nanotubes of carbon and other materials are in vogue for altering the properties of materials, for example improving mechanical toughness or adding electrical conductance. Composites of nanoparticles and fibres should provide new functional materials, perhaps

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enhancing common polymers such as polyester or polyethylene by imparting fire retardance or photocatalytic behaviour or antimicrobial properties. Although wet spinning or electrospinning are current methods for combining these two types of matter, CVD could offer an alternative route that does not require the nanoparticles to be separately synthesised before incorporation into the composite, and at the same time avoiding the handling of volumes of liquid waste. Military needs are driving research into interactive or intelligent fabrics, but there are related civilian uses. Incorporating electronics, sensors and power sources into fabrics is an obvious requirement for infantry clothing, but also has potential uses in monitoring sporting performance or medical treatments. PPy was discussed above as a conducting polymer, but when it is prepared accordingly it may also indicate mechanical strain through the variation in its electrical resistance. Aerogels are another structural variation on existing materials that offer lowdensity materials with a large surface area and greatly enhanced properties such as high thermal insulation. By combining this structure with functionalised surfaces, new intelligent materials may be fabricated that contain both sensors and active response to the signals, perhaps including biomedical functions. It is clear that advances in textile technology are now demanding a combined approach from physics, chemistry, materials science and biochemistry to design and fabricate more sophisticated products.

6.6

Further sources of information

Chemical Vapor Deposition, Principles and Applications, by M L Hitchman and K F Jensen, Academic Press, New York, 1993, ISBN 0-12-349670-5: presents a general overview of the principles and uses of CVD. Process Engineering Analysis in Semiconductor Device Fabrication, Chapter 12, Chemical vapor deposition, by S Middleman and A K Hochberg, McGraw-Hill, New York, 1993, ISBN 0-07-041853-5: addresses both physics and chemistry aspects of reactor design in some depth for semiconductor device materials but is relevant to other coatings.

6.7

References

A C ADAMS (1983), Dielectric and polysilicon film deposition, Chapter 3 in VLSI Technology, ed. S M Sze, McGraw-Hill, Japan, pp. 93–129. M ASANO, M INOUE, Y TAKAI, T MIZUTANI AND N IEDA (1988), Preparation of polypyrrole by CVD and its electrical properties, in Proceedings of the 21st Symposium on Electrical Insulating Materials, pp. 219–222. G COOK, P L TIMMS AND C GOLTNER-SPICKERMANN (2003), Exact replication of biological structures by chemical vapour deposition of silica, Angew. Chem. Int. Ed., 42, 557–559. G K HYDE, K J PARK, S M STEWART, J P HINESTROZA AND G N PARSONS (2007), Atomic layer deposition of conformal inorganic nanoscale coatings on three-dimensional natural fiber systems: effect of surface topology on film growth characteristics, Langmuir, 23, 9844–9849.

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M-Y S LEUNG, J TSANG, X-M TAO, C-W M YUEN AND Y LI (2006), Stability enhancement of polpyrrole coated textiles, Chapter 15 in Intelligent Textiles and Clothing, ed. H R Mattila, Woodhead Publishing Ltd, Cambridge, UK. S LI, H XIE, S ZHANG AND X WANG (2007), Facile transformation of hydrophilic cellulose into superhydrophobic cellulose, Chem. Commun., 4857–4859. S LUO AND W J VAN OOIJ (2002), Surface modification of textile fibers for improvement of adhesion to polymeric matrices: a review, J Adhesion Sci. Technol., 16, 1715–1735. M MA, Y MAO, M GUPTA, K K GLEASON AND G C RUTLEDGE (2005), Superhydrophobic fabrics produced by electrospinning and chemical vapor deposition, Macromolecules, 38, 9742–9748. T P MARTIN, S E KOOI, S H CHANG, K L SEDRANSK AND K K GLEASON (2007), Initiated chemical vapor deposition of antimicrobial polymer coatings, Biomaterials, 28, 909–915. Y XU, L CHENG, L ZHANG, X YIN AND H YIN (2001), High performance 3D textile HiNicalon SiC/SiC composites by chemical vapour infiltration, Ceramics Int., 27, 565–570.

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M-Y S LEUNG, J TSANG, X-M TAO, C-W M YUEN AND Y LI (2006), Stability enhancement of polpyrrole coated textiles, Chapter 15 in Intelligent Textiles and Clothing, ed. H R Mattila, Woodhead Publishing Ltd, Cambridge, UK. S LI, H XIE, S ZHANG AND X WANG (2007), Facile transformation of hydrophilic cellulose into superhydrophobic cellulose, Chem. Commun., 4857–4859. S LUO AND W J VAN OOIJ (2002), Surface modification of textile fibers for improvement of adhesion to polymeric matrices: a review, J Adhesion Sci. Technol., 16, 1715–1735. M MA, Y MAO, M GUPTA, K K GLEASON AND G C RUTLEDGE (2005), Superhydrophobic fabrics produced by electrospinning and chemical vapor deposition, Macromolecules, 38, 9742–9748. T P MARTIN, S E KOOI, S H CHANG, K L SEDRANSK AND K K GLEASON (2007), Initiated chemical vapor deposition of antimicrobial polymer coatings, Biomaterials, 28, 909–915. Y XU, L CHENG, L ZHANG, X YIN AND H YIN (2001), High performance 3D textile HiNicalon SiC/SiC composites by chemical vapour infiltration, Ceramics Int., 27, 565–570.

7 Enzyme surface modification of textiles V. A. NIERSTRASZ Ghent University, Belgium

Abstract: The chapter discusses recent progress and new developments in the enzymatic surface modification of textiles. Enzymatic surface modification of textile materials involves processing of fibres or (bio)polymers to modify the physical and chemical surface properties or the introduction of functional groups on the surface. In this chapter the focus is on cellulases, pectinases, xyloglucan endotransglycosylase, tyrosinases and cutinases in the surface modification of textile materials to demonstrate the potential and different applications of enzyme technology in this area. Research on the enzymatic surface modification of textiles contributes to biobased economy through the development of novel processes for textiles exhibiting targeted functionalities and through development of novel enzyme technology for structuring and functionalisation of surfaces. Key words: biotechnology, enzymes, surface modification, textile materials, functionalisation.

7.1

Introduction: principles of enzyme surface modification of textile materials

Textiles are extremely versatile materials, combining different fibres and structures for a wide range of applications. Besides the well-known applications, textiles are used in medical applications (e.g. wound treatment, sensors, artificial aortas and vascular grafts), but are also used abundantly in, for example, sport, vehicles, geo-textiles, aircraft, construction, agriculture, defence, transport, packing/storage and the chemical industry. Today, the textile industry is typically in a transformation from labour-intensive products to knowledge-intensive products and therefore the textile industry has a great demand for innovative hightech materials with special properties and added functionalities. Often special properties or functionalities are achieved through the manipulation of the surface characteristics of the textile material (Fig. 7.1). Well-known examples include: water-repellent textiles that are permeable to air and allow liquid flow in only one direction; plasma treatments affecting, for example, wetting properties; or soiland water-repellent coatings affecting the overall performance. These approaches do not make use of all the physico-chemical possibilities; one can envision the application of new paradigms and design principles found in or inspired by natural 139

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Enzymes ENZYMES Functionality

Material

Surface engineering

Synthetic Natural Biomaterial

Characteristics

7.1 Enzymatic surface modification and functionalisation of textiles.

systems. The complex relationship of various key characteristics allows various organisms, for instance, to outperform many man-made surfaces. An illustration of the effectiveness of such an advanced functional material in nature is the watercapturing surface of the beetle Sternocera (Parker and Lawrence, 2001). This beetle is able to collect water from fog via a combination of surface roughness and chemical heterogeneity (a topo-chemical effect). Another example of the strength of nature in modifying surface properties is the production of hydrophobins by fungi; this affects surface properties and thereby enables fungi to pass the air– water interface (Scholtmeijer et al., 2004; Wosten and de Vocht, 2000). Such an approach, based on bio-inspired functionalised surfaces such as hydrophobins, could lead to new functional biomedical textile materials. Why explore enzyme surface modification of textile materials? Manipulation of the surface characteristics of textile materials is of fundamental importance in the production of advanced (functional) textiles. The development of biotechnologically functionalised textile materials requires improved fundamental understanding of the relevant parameters and the development of suitable application techniques and processes. While much research focuses on chemical modification or structuring of the surfaces, the introduction of functionalities using enzymes is a relatively unexplored scientific area. The advantage of biotechnology, or more specifically enzymes, over other technologies is their high specificity towards a certain reaction and/or substrate. In addition to that, research efforts are seldom focusing on the combined effect of chemical heterogeneity, topography and roughness caused by the surface treatment (topo-chemistry). Biotechnology, or more specifically enzyme technology, has enormous potential for the production and synthesis of textile materials with advanced

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functionalities because of the high specificity of enzymes. The importance and potential of enzyme technology in textiles has been assessed in recent years (for recent reviews see, for example, Araújo et al. (2008), Gübitz and Cavaco-Paulo (2003) and Lenting (2004)). Enzyme technology has already proven to be very profitable in industrial textile pre-treatment processes of natural fibres; however, application of enzymes is not limited to natural materials, recently it has been demonstrated that enzymes are able to modify the surfaces of synthetic textile materials as well (for a recent review see Gübitz and Cavaco-Paulo (2008)). A variety of chemical techniques exist for the production of textile fibres bearing functional molecules at their surfaces. Often these chemical techniques lack the specificity of biotechnological systems, and often these techniques are less attractive from an environmental or economic point of view. Taking into account the current international state of the art, today’s scientific challenge is to make the enormous potential of biotechnology for the production and synthesis of textile materials with advanced functionalities and surface properties an opportunity for the textile industry. In this chapter the focus will be on more conventional, state of the art, enzyme applications as well as some novel promising enzymatic systems for the surface modification of textile materials. Over the last two decades, much research has been carried out on reactions catalysed by enzymes that are relevant for the textile industry; for example, desizing of cotton, depilling of cotton, enzymatic ageing of denim fabrics, enzymatic scouring of greige cotton fabrics, shrink-resistant treatment of wool and enzymatic degumming of silk. These studies clearly indicate that the (surface) modification of textile polymers with enzymes has the potential to replace conventional chemical processes. Progress in biochemistry, biocatalysis, genetics and largescale production of enzymes makes industrial processes using enzymes more attractive and competitive. Consequently, biocatalysis plays an increasingly important role in industrial wet textile pre-treatment and finishing processes. Conventional wet textile processes require long residence times, high concentrations of harsh chemicals, large amounts of water and comprise several high-temperature operations leading to high energy consumption. It is to be expected that wet textile processes will be shifted considerably towards benign processes based on biocatalysis, owing to increasing governmental and environmental restrictions and the decreasing availability of fresh water. Biotechnology is flexible and reliable, and a promising technology to fulfil the expected future requirements. New enzyme systems and novel processes are explored and developed for the treatment and (surface) modification of typical textile fibres (synthetic as well as natural fibres) – for example cotton, flax, hemp, nettle, polyesters and polyamides, and less conventional or ‘new’ textile fibres (synthetic as well as natural fibres) such as chitin/chitosan, poly(lactic acid) (PLA) – in order to modify or to improve (surface) properties, to introduce functionality, to biomimic specific properties, to increase performance or to increase the applicability of such fibres and to facilitate

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processing. The global challenge of decreasing pollution levels and water consumption is a stimulant for the production of textile fibres from renewable resources. Enzymatic oxidation and enzymatic hydrolysis of the surfaces of synthetic fibres such as polyesters and polyamides has been reported fairly recently in the scientific literature. Oxidation or hydrolysis not only affects fibre properties such as hydrophilicity but also facilitates functionalisation, modification or crosslinking of the polymers. Because of the different nature, requirements and characteristics of enzyme systems compared with regular chemistry or catalysis, the focus needs to be on the application of the enzyme systems using innovative processes, to increase competitiveness and efficiency by minimising water and chemical consumption. This will allow the implementation of advanced biotechnological processes in the textile industry.

7.2

Enzymes, technologies and materials (natural materials, synthetic materials, biomaterials)

Enzymatic surface modification of textiles involves processing of fibres or (bio)polymers to increase hydrophilicity or to remove components from the surface through hydrolysis, oxidation or eliminative cleavage as well as the (targeted) introduction of functional groups on the surface. In this section, the focus is on the use of cellulases, pectinases, xyloglucan endotransglycosylase, tyrosinases and cutinases for the surface modification of textile materials to demonstrate the potential and different applications of enzyme technology in this area.

7.2.1

Cellulases in the surface modification of cotton, Tencel, viscose and linen

Cellulases are the most accepted and successful application of enzymes in the textile industry. Cellulases are hydrolytic enzymes that find application in several different (textile) processes. Among other applications, both within and outside the textile industry – such as biofuels and enzymatic ageing of denim – cellulases find application in the removal of microfibrils from the surface of cellulosic materials such as cotton, Tencel (Lyocell), viscose and linen (Azevedo et al., 2000; Buschle-Diller et al., 1994; Carrillo et al., 2003; Cavaco-Paulo, 1998; Ciechañska et al., 2002; Cortez et al., 2001; Heikinheimo, 2002; Kan et al., 2008; Kumar et al.,1997; López-Lorenzo, 2004; Pere et al. 2001; Valldeperas et al., 2000). Cellulase treatment results in a cleaner fabric surface with less fuzz, a reduced tendency to pill formation, clearer colours, a reduced amount of dead and immature cotton, a softer fabric and improved drapeability. The softening effects are wash-proof and non-greasy, in contrast to conventional chemical softening

Enzyme surface modification of textiles

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7.2 Cellulose hydrolysis.

agents. In nature, cellulosic materials are hydrolysed (Fig. 7.2) by the synergistic action of three general types of cellulases (Cavaco-Paulo and Gübitz, 2003; Teeri, 1997): • endo-β-1,4-glucanase (EC 3.2.1.4), also named cellulase or endoglucanase; • cellulose 1,4-β-cellobiosidase (EC 3.2.1.91), also named cellobiohydrolase (CBH), exo-cellobiohydrolases or exoglucanase; • β-glucosidase (EC 3.2.1.21), also named cellobiase. Endoglucanases hydrolyse amorphous cellulose randomly into smaller fragments, thereby generating new ends. Exoglucanases act on the ends of the cellulose polymer, thereby generating cellobiose. Cellobiose is hydrolysed by β-glucosidase, thereby producing glucose. Exoglucanases are thought to act on more crystalline regions as well, thereby making them more susceptible to hydrolysis by endoglucanases. There are two main types of exoglucanases, one acting progressively from the non-reducing end of the cellulose polymer, and one acting progressively from the reducing end of the cellulose polymer (Fig. 7.3). The classification of cellulases into endoglucanases and exoglucanases is not strict, since cellulases have been found displaying endoas well as exo-activity. Hydrolysis of the cellulose polymer is via the β-β retaining mechanism, or via the β-α inverting mechanism (Fig. 7.4). Cellulases can be obtained from a variety of micro-organisms, but particularly from fungi such as Trichoderma reesei, Humicola insolens, Aspergilus niger and the bacterium Bacillus subtilus. Micro-organisms produce (excrete) cellulases to obtain glucose. Natural mixtures of cellulases are very effective in converting cellulose into glucose. In textile finishing processes this is not necessary and may even be unwanted. Traditionally the performance of cellulases was classified according to their optimum temperature or pH (acidic stable, neutral or alkali stable). Today specific formulations are available commercially, designed for specific applications or effects, e.g. conventional mixtures, endoglucanaseenriched formulations, monocomponent cellulases and modified cellulases with unique properties such as increased temperature stability. Depilling (also called biopolishing) of cellulosic fabrics is the removal of fuzz (microfibres) from the surface. During different processes, these microfibres

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Non-reducing end

Reducing end

CH2OH OH OH

7.3 Cellulose polymer, reducing and non-reducing ends.

become entangled and pills are formed. Enzymatic depilling, an effective process to remove these pills, is a process to enhance the quality of fabrics. Endoglucanaseenriched or endoglucanase monocomponent formulations are usually preferred because of their superior performance. Enzymatic depilling requires equipment with sufficient shear forces and mixing such as a jet or a winch. Continuous wet textile processing equipment does not produce sufficient shear forces and mixing for enzymatic depilling. The cellulase enzymes locally weaken the fibres protruding from the surface by the preferential degradation of the amorphous parts of the fibre. The enzyme-weakened fibres are sensitive to shear forces, and upon application of sufficient shear the fibre will break from the fabric surface (Cavaco-Paulo et al., 1996, 1997; Lenting and Warmoeskerken, 2001). Depilling can be carried out after any textile pre-treatment process but is preferably carried out after the processes that generate fibrils and pills, e.g. in jets and wash drums, and if possible after bleaching and dyeing; the cellulase treatment can result in partial dye removal, and hence colour differences. Typical process conditions and auxiliaries are described by, for example, Nierstrasz and Warmoeskerken (2003). The cellulases need to be inactivated after the depilling process; insufficient inactivation will result in extended degradation of cellulose and therefore an undesirable decrease in the degree of polymerisation (DP), strength loss and weight reduction.

7.2.2

Pectinases in the enzymatic scouring of cotton

Pectinolytic enzymes are among the most important industrial enzymes with (potential) applications in cotton scouring, flax processing (retting) and fruit juice clarification. Like cellulases, pectinases are widespread in nature. Pectinases are produced in different micro-organisms as well as in plants. Pectinases are a group of enzymes (lyases as well as hydrolases) that catalyse the degradation of pectin, and are classified into three groups: pectin esterases, polygalacturonases and

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145

(a) O

OH RO O HO

O

OH

RO

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H

OR O OH

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–

OH

H

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O

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RO

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OH OR O

OH

H O H O

O

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O

OH RO

HO

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H

HO OH O

HO O

OR O

OR O OH

H O

7.4 Hydrolysis of the cellulose polymer via the β–β retaining mechanism (a), or via the β–α inverting mechanism (b).

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pectin lyases (Hoondal et al., 2002; Sakai et al., 1993). Pectin esterase (EC 3.1.1.11) de-esterifies polymethyl-galacturonate forming polygalacturonate. Polygalacturonase hydrolyses α-(1–4) linkages in polygalacturonate, and can be divided in two groups according to the following criterion: whether cleavage occurs randomly within the polymer (endopolygalacturonase, EC 3.2.1.15) or end-wise from the non-reducing end of the polymer (exopolygalacturonase, EC 3.2.1.67). Pectin lysases remove double bonds in polygalacturonate via β-elimination, thereby cleaving the polymer. Three types of pectin lyases exist, namely: endopolygalacturonate lyases (EC 4.2.2.2) which randomly cleave polygalacturonate; exopolygalacturonate lyases (EC 4.2.2.9) which cleave polygalacturonate chains at the ends yielding unsaturated galacturonic acid; and endopolymethylgalacturonate lyases (EC 4.2.2.10) which randomly cleave pectin (Sakai et al., 1993). With increasing knowledge and understanding of pectinolytic enzymes and of their different pectin degradation mechanisms, alkaline pectate lyases (EC 4.2.2.2) seem to have great potential in cotton scouring (e.g. Buchert et al., 2000; Etters, 1999; Hartzell-Lawson and Hsieh, 1998; Lange, 2000; Li and Hardin, 1998; Lu, 2005) and enzymatic flax retting (Akin et al., 2007). Actual commercialisation of enzymatic cotton scouring is still limited because of the relatively long incubation time, residual pectin content and the processing temperature required (~60 ºC). The enzymatically scoured fabrics have a softer hand than those scoured conventionally, however the degree of whiteness is often somewhat lower and the process is not yet suitable for adequate removal of seed coat fragments and motes. The purpose of scouring is to clean the surface of cotton fibres and to remove hydrophobic and non-cellulose parts of the cotton, rendering the surface of the fibres sufficiently hydrophilic. These hydrophobic components and non-cellulosic materials present in the cuticle and the primary wall of cotton have a role in the protection of the plant from the environment and pathogens as well as a structural role. In the conventional process, hydrophobic materials are removed in a nonspecific manner and this involves the use of relatively harsh chemical solutions and high temperature (Nierstrasz and Warmoeskerken, 2003). Much research has been directed to replace hydroxide scouring with an environmentally benign enzymatic process (Agrawal et al. 2007, 2008a, 2008b; Buchert et al., 2000; Buschle-Diller et al., 1998; Choe et al., 2004; Csiszár et al., 2001; Degani et al., 2002; Etters, 1999; Hartzell-Lawson and Hsieh, 1998; Kalantzi et al., 2008; Klug-Santner et al., 2006; Lange, 2000; Lenting et al., 2002; Li and Hardin, 1998; Lu, 2005; Sawada et al., 1998; Tzanov et al., 2001; Wang et al., 2007; Yachmenev, 2001). Research has been carried out to investigate the potentials of different enzymes – such as pectinases, cutinases, cellulases and lipases, as well as combinations thereof – and different process conditions to degrade and to remove the undesired components from the cotton fibres. Among others, Novozymes and Tanatex have proposed an enzymatic alternative for the industrial scouring of woven and knitted cotton fabrics in the

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textile industry on the basis of an alkaline pectate lyase (EC 4.2.2.2) produced by a genetically modified Bacillus strain; other companies have proposed alternatives based on mixtures or multicomponent formulations. This industrial enzymatic scouring process, based on alkaline pectate lyases, has been performed successfully in batch (pad-batch) processes (Lange, 2000). Typical process conditions and auxiliaries are described by, for example, Nierstrasz and Warmoeskerken (2003). The strategy is that the pectin matrix stabilises the primary cell wall of the cotton fibres. The pectate lyases will destabilise the structure in the outer layers so the weakened outer layers can be removed in a subsequent wash process (Li and Hardin, 1998) with sufficient shear (mechanical action). However, owing to the relatively long processing time, enzymatic scouring has not been widely implemented. The most important reason identified is the inability to remove cotton waxes during scouring with pectate lyases (Agrawal et al., 2007; Lu, 2005); the cuticle hinders the performance of the enzyme as well as mass transfer. Cutin forms three-dimensional-network structures and in this network amorphous waxy materials are embedded (Kolattukudy, 2001; Purdy and Kolattukudy 1975). A treatment to remove waxes at laboratory scale, such as prerinsing in hot water (T > 90 ºC) with a surfactant (Hartzell-Lawson and Hsieh, 1998) or extraction with n-hexane, helps to reduce wax content and, subsequently, results in a better pectinase performance (Agrawal et al., 2007). This demonstrates the importance of removing the waxes effectively. The main challenge hindering the successful industrial implementation of enzymatic cotton scouring is the removal of these cotton waxes effectively and efficiently at low temperature via an environmentally benign route. The potential of cutinase (EC 3.1.1.74) for industrial cotton wax degradation was assessed by Degani et al. (2002) and Agrawal et al. (2008a, 2008b). Degani et al. (2002) were the first to report on the potential of cutinase from a bacterial source, Pseudomonas mandocino, for wax degradation in cotton scouring; however, the incubation time was still between 10 and 20 hours. Agrawal et al. (2008a, 2008b), using a cutinase from Fusarium solani pisi, were able to meet the desired benchmarks (contact angle as well as pectin content comparable with the conventional treatment) in less than 15 minutes at low temperature, through the removal of waxes using cutinases and the synergistic action of alkaline pectate lyases and mechanical action. The major hurdles that prevented the introduction of a viable (continuous) industrial enzymatic scouring process are actually overcome by this strategy.

7.2.3

Xyloglucan endotransglycosylase (XET) in the chemo-enzymatic surface modification of cellulosic materials

Recently Brumer et al. (2004) developed a chemo-enzymatic method, inspired by nature, for the modification of cellulosic materials based on XETs (EC 2.4.1.207),

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a group of enzymes from the glycoside hydrolase family. Using their approach a wide range of functionalities can be attached onto cellulosic surfaces (Baumann, 2007; Teeri et al., 2007; Zhou et al., 2005, 2006, 2007). Instead of the surface modifications discussed in the paragraphs above, this functionalisation is not through enzymatic degradation, but through enzymatic synthesis. Xyloglucans are polysaccharides with a highly xylose substituted β-(1–4)linked glucan backbone. In plants, xyloglucans have a role as storage as well as cross-linking agents. Xyloglucans associate tightly, through adsorption or entrapment, with cellulose microfibrils. The cellulose microfibrils are linked via xyloglucan tethers. Thereby they contribute to the extensibility of the primary cell wall, as well as its porosity. During plant cell growth these networks must be loosened temporarily and reinforced afterwards; XETs play a key role in this process. XETs are enzymes active in the primary cell wall of plants. XETs are enzymes that are able to rearrange xyloglucan polymers by cleavage of β-(1–4) bonds in the backbone of xyloglucans, and subsequent transfer of the xyloglucanyl segment onto the O–4 of the non-reducing terminal glucose residue of the xyloglucan polymer. In addition to xyloglucan polymers, XETs are able to incorporate xylogluco-oligosaccharide derivates into xyloglucans via a transglycosylation reaction even if they are chemically modified. This allows specific functionalisation of cellulosic materials. Soluble xyloglucan oligosaccharides are produced from xyloglucans from tamarind seeds by hydrolysis using microbial xyloglucanases (EC 3.2.1.151). These xyloglucan oligosaccharides are chemically modified by reductive amination. The aminoalditol derivate of the xyloglucan oligosaccharides serves as a key intermediate. This reactive amino group can be specifically derivatised with different functional groups. The xyloglucan oligosaccharides can be incorporated in the high molecular weight xyloglucans by an endotransglycosylation reaction catalysed by XET from Populus tremula × tremuloides, to produce aminated xyloglucans. These aminated xyloglucans have high affinity for cellulose (Fig. 7.5). The amount of xyloglucan adsorbed onto cellulose is typically between 3.5 and 7.5 mg/g in 24 hours (Brumer et al., 2004). The reactive amino group can now be functionalised. Alternatively, the aminated xyloglucan oligosaccharides are derivatised with the required functional group first, followed by the XETcatalysed endotransglycosylation with xyloglucans and subsequent adsorption to the cellulosic material. Using this method a number of different functionalities were developed by Brumer and co-workers such as: amino groups, fluorescent dyes (fluorescein isothiocyanate and sulforhodamine), cinnamoyl derivates that can be activated by visible light or ultraviolet, biotin, optical brightening agents or polymers (for example PLA) (Zhou et al., 2007). The technology, which is still in the laboratory phase, seems to have potential for the production of advanced functionalities with applications in textiles, paper and packaging, as well as other cellulosic materials. The technology relies on xyloglucans, a renewable material readily available from

Enzyme surface modification of textiles Reductive amination Xyloglucan oligosaccharide

NH2

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R R NH NH

R R NH NH R R NH NH

R R NH NH R R NH

R R NH NH

R R NH NH R R NH NH

R R NH NH

Cellulose cellulose Functionalised xyloglucan

7.5 Chemo-enzymatic functionalisation of cellulose as developed by Brumer et al. (2004).

agriculture, in combination with chemo-enzymatic processes. Xyloglucans are already used in the textile industry as sizing agents for the production of denim (Kalum, 1998). Development and scaling-up of production and downstream processes for XET are important topics for industrial implementation of this innovative technology (Henriksson, 2007).

7.2.4

Tyrosinase in the enzyme surface modification of chitosan and protein fibres

Tyrosinases (EC 1.14.18.1) are copper-containing enzymes that are widely distributed in nature (e.g. they are produced in mushrooms) and that find applications in textiles in the coupling of functional molecules containing a phenol group to protein fibres (wool and silk) or to other fibres with a free amino group such as chitosan (e.g. Anghileri et al., 2007; Freddi et al., 2006; Jus et al., 2008; Kumar et al., 1999, 2000; Muzzarelli and Ilari, 1994; Muzzarelli et al., 1994; Sampaio et al., 2005). Tyrosinases convert solvent-exposed tyrosine residues of various proteins as well as other phenolic substrates to o-quinones through binding of O2 (Fig. 7.6). The electrophilic o-quinones can react with a variety of nucleophilic amino groups in proteins or chitosan (Fig. 7.7). Chitosan is a biopolymer with special properties. Chitosan is derived from chitine, after cellulose the most abundant polysaccharide in nature. Chitosan has antibacterial properties and it has medical applications, e.g. in the treatment of burns or decubitus. Tyrosinase-catalysed reactions are explored practically to confer relevant functionalities on chitosan. For example, Kumar et al. described the modification of chitosan through enzymatic grafting of chlorogenic acid onto chitosan (Kumar et al., 1999), as well as through enzymatic grafting of p-cresol

150

Surface modification of textiles OH

O O

Tyrosinase

+ H2O + O2 R

R

7.6 Reaction schema of tyrosinase, formation of o-quinone. R represents a protein or any functional group.

O

H N

O O

NH2

R

+ Schiff base R

O

Chitosan or protein

O

N H

R

Michael-type adduct

7.7 Reaction of o-quinone with an amino group of, for example, chitosan or a protein.

onto chitosan (Kumar et al., 2000). The purpose of grafting chlorogenic acid onto chitosan is to increase solubility at higher pH. Above pH 6.5 the amine groups of chitosan are deprotonated, and chitosan is insoluble; by enzymatic treatment at pH 6 it was possible to modify chitosan under homogeneous conditions. Incorporation of p-cresol in chitosan was achieved by incubating chitosan films at pH 6.5 for 1–2 hours with tyrosinase (20 U/ml) and p-cresol (1.8 mM). Wu et al. (2001) used tyrosinase-coated chitosan films to remove volatile phenols, such as p-cresol, from air. The potential of tyrosinase in the enzymatic grafting of chitosan on to silk fibroins (in homogeneous and heterogeneous systems) or the enzymatic grafting of sericin peptides on to chitosan, has been explored by Anghileri et al. (2007), Freddi et al. (2006) and Sampaio et al. (2005). They demonstrated that tyrosinase could oxidise 57% of the tyrosine residues of sericin and ~10% of the tyrosine residues

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of regenerated silk gels, but the yield of the reaction was low for silk powder and undetectable for fibres, although by Fourier transform (FT)-Raman spectroscopy oxidation by tyrosinase could be confirmed. FT-infrared and FT-Raman spectroscopy confirmed the formation of silk–chitosan bioconjugates under heterogeneous reaction conditions, although chitosan grafting on silk fibres could not be demonstrated. These results can be explained on the basis of accessibility of tyrosyl groups. The loose three-dimensional arrangement of regenerated silk fibroin chains in the homogeneous system allows swelling in the reaction medium, thereby increasing the accessibility of tyrosyl groups; in contrast, the compact, highly ordered and oriented fibrous structure of the silk powder and fibres does not swell and thus only allows oxidation of tyrosyl groups at the surface. These researchers have demonstrated the potential of tyrosinasemodified or -functionalised (regenerated) silk materials. Jus et al. (2008) studied the enzymatic functionalisation of wool using tyrosinases by grafting of phenolic antioxidants (caffeic acid and chlorogenic acid). Tyrosinase oxidises tyrosine residues in wool proteins to o-quinones, which can react with caffeic acid and chlorogenic acid. Ascorbic acid was added to increase the accessibility of tyrosine in wool and to increase the conversion. Jus et al. (2008) successfully demonstrated that the antioxidants are strongly cross-linked to the wool fibre, and that the enzymatically functionalised wool had improved antioxidant activity, 75.2% and 51.4% for caffeic acid and chlorogenic acid respectively. Wool functionalised with antioxidants could have potential applications in innovative functional textile materials protecting the skin. The strength of tyrosinase technology lies in its simplicity and the wide range of substrates that can be functionalised or modified with a broad range of functional groups. Polymer modification or functionalisation is simple and rapid compared with other approaches, although the o-quinone reactions are difficult to control.

7.2.5

Cutinase in the enzyme surface modification of poly(ethylene terephthalate) (PET)

Cutinases (EC 3.1.1.74) are serine hydrolases specific for the hydrolysis of cutin. Cutin is a polymer in the cuticle of higher plants; it is a biopolyester composed of hydroxyl and epoxy fatty acids (Kolattukudy, 2001; Purdy and Kolattukudy, 1975). The hydroxy fatty acids are usually C16 or C18 and contain up to three hydroxyl groups. Cutinases are extracellular esterases mainly produced by pathogenic fungi and pollen, although some cutinases are produced in bacteria such as Pseudomonas putida and a Corynebacterium sp. (for a review on cutinases see Carvalho et al. (1999). Cutinase from Fusarium solani pisi is the most studied cutinase. This is a one-domain molecule, i.e. there is no quaternary structure; it is relatively small, consisting of an amino acid sequence of 197 residues with a weight of 22 000 g/mol; it is approximately 45 × 30 × 30 Å3 in size (Carvalho et al., 1998). Cutinase belongs to the family of serine esterases and is a member of the

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superfamily of α/β hydrolases like lipases (EC 3.1.1.3) and carboxylesterases (EC 3.1.1.1). Carboxylesterases, lipases (triacylglycerol acylhydrolase) and cutinases are carboxylic ester hydrolases (EC 3.1.1), and members of the esterases (EC 3.1) – enzymes that hydrolyse esters into an acid and an alcohol. The catalytic triad Ser-Asp-His is accessible to the liquid, there is no lid (as in lipases), and the oxyanion hole is preformed but considerably flexible in solution, in contrast to lipases. Also in contrast to lipases, cutinases do not require interfacial activation. In cutinase, the oxyanion hole stabilises intermediates and lowers the activation energy for catalysis. Because of the flexible binding loop and the oxyanion hole, cutinase can accept a wide range of substrates, although conversion rates differ depending on the substrate. Synthetic fibres form an important part of the textile industry, the production of polyester alone surpassing that of cotton. In 2007 the world production (fibres and yarns) was 30.7 million tonnes of polyester, 4.0 million tonnes of polyamide, 2.4 million tonnes of acrylics and 26.1 million tonnes of cellulose (Oerlikon, 2008). PET fibres are used in a wide range of applications, e.g. in advanced applications such as biomedical textiles. The aim of the enzymatic surface modification of PET is to increase hydrophilicity locally, allowing specific functionalisation. The advantage of enzymes over conventional techniques is that the favourable bulk properties of PET are not affected because the enzymes are too big to penetrate into the bulk phase of the material. Over the last few years much research effort has been focused on the potential of lipases, polyesterases and cutinases for the modification, and eventually functionalisation, of PET. Synthetic materials have generally been considered resistant to biological degradation, it was not until the last 5–6 years that evidence has been presented of the enzymatic hydrolysis of PET by cutinases and (poly)esterases, as well as some lipases (Fig. 7.8) (e.g. Alisch-Mark et al., 2004, 2006; Brueckner et al., 2008; Hsieh and Cram, 1998; Müller et al., 2005; Nimchua et al., 2006; Silva et al., 2005; Vertommen et al., 2005; Yoon et al., 2002) as well as oligomers of PET (Hooker et al., 2003; Nechwatal et al., 2006). Cutinases not only find application in the modification of PET, it has also been demonstrated recently that cutinases are effective in the surface modification of polyamide 6,6 fibres (Silva and Cavaco-Paulo, 2004, 2008; Silva et al., 2005); polyamidases and proteases hydrolysing polyamide 6,6 are also reported to have this capability (Fig. 7.8) (Almansa et al., 2008a; Silva et al., 2007a,b). Cutinases from Fusarium solani pisi, Fusarium oxysporum and Thermobifida fusca are frequently studied and seem to be promising enzymes for the enzymatic surface modification of PET. There are currently quite some research efforts in this rapidly developing area (Almansa et al., 2008b; Donelli et al., 2008; Feuerhack et al., 2008; Kim and Song, 2006; Liebminger et al., 2007; Liu et al., 2008; Nimchua et al., 2008; O’Neill et al., 2007, Silva and Cavaco-Paulo, 2008), focusing on the modification of PET yarns, fibres, fabrics and films, as well as the use of enzymes from other sources. A variety of analytical methods is used to assess the

Enzyme surface modification of textiles

153

PET O O C

O

CH2 C O

O C

CH2

O

CH2 C O CH

O C

2

CH2

O

O

n

Cutinase, lipase or polyesterase

Polyamide H

H

H

H

H

O

N

C

C

C

C

C

C

H

H

H

H

H

H

n

H

H

H

H

H

O

N

C

C

C

C

C

C

H

H

H

H

H

H

Cutinase, amidase or protease

7.8 Hydrolysis of PET and polyamide.

modifications, for example high-performance liquid chromatography (HPLC) (detection of hydrolysis products (oligomers/monomers) in solution) (Vertommen et al., 2005; Yoon et al., 2002), differential scanning calorimetry (DSC) (Vertommen et al., 2005), scanning electron microscopy (SEM) (Feuerhack et al, 2008; Kim and Song, 2006), FT-infrared (Donelli et al., 2008), electron spectroscopy for chemical analysis/X-ray electron spectroscopy (ESCA/XPS) (Vertommen, et al., 2005), contact angle (Donelli et al., 2008), dyeing assay (determining the increase of hydroxilic groups by reactive dye) (Alisch-Mark et al., 2004; O’Neil and Cavaco-Paulo, 2004), spectroscopic assays to determine the release of terephthalic acid (TPA) (O’Neil and Cavaco-Paulo, 2004), liquid uptake of fabric (rising height) (Alisch-Mark et al., 2006) and weight loss (Kim and Song, 2006; Müller et al., 2005). Care must be taken with measurements of changes in surface properties because cutinase strongly absorbs onto the surface. Washing, extraction or protease procedures have been proposed and developed to remove cutinase from the PET surface. Crystallinity greatly affects the capability of the enzyme to hydrolyse the ester bonds; the enzymes display relatively high activity towards an amorphous polyester film and little activity on a highly crystalline substrate (Müller et al., 2005; Vertommen et al., 2005). The ratio of hydrolysis products formed (TPA, mono(2 hydroxyethyl) terephthalate and bis(2 hydroxyethyl) terephthalate) depends on the source of the enzyme, the substrate, enzyme–substrate ratio, enzyme concentration and incubation time (Hooker et al., 2003; Vertommen et al., 2005). This means that the use of model substrates does not necessarily give sufficient information for the actual modification of PET.

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Surface modification of textiles

Building 3D structures/devices

Specific enzymatic surface modification

Enzymatic synthesis of functional groups

7.9 Inkjet technology in the enzymatic surface functionalisation of textiles.

Cutinase from Fusarium solani pisi has an optimum pH around 8–8.5 and an optimum temperature around 25 ºC, above 35 ºC the activity rapidly decreases. Enzymatic PET treatment results in significant depilling, increased hydrophilicty, increased reactivity with cationic dyes, and only a small decrease in weight. The enzymatic process is rather slow; this hinders the use of cutinase in PET recycling or for modification of entire fabric surfaces but for specific surface modifications and functionalisation cutinases have clear benefits. SEM and environmental scanning electron microscopy (ESEM) images of enzymatically treated PET show a more or less homogeneous surface treatment, while in conventional NaOH-treated PET pitting corrosion is visible (Brueckner et al., 2008; Kim and Song, 2006). Brueckner et al. (2008) demonstrated that in the conventional NaOH treatment no additional superficial groups are formed, whereas after enzymatic treatment novel superficial carboxyl and hydroxyl groups are formed. This demonstrates the potential of the enzymatic technique for the functionalisation of PET. Control of enzymatic action in the correct time and length scales is a prerequisite for achieving the desired functionalities. Therefore the potential of sophisticated non-contact dispensing technologies such as softlithography and inkjet technology are explored in order to design novel production processes for textiles that exhibit the desired targeted functionalities (Fig. 7.9). Inkjet technology allows specific surface modification, and allows functionalisation and reactions at specific locations, allowing the design of highly specific localised functionalities. In order to improve the performance and applicability of cutinase, temperature stability needs to be increased and the reaction rate enhanced. Increasing temperature stability would allow working at higher temperatures, which has the advantage of increased mobility of the polymers and thus increased accessibility of the polymers to the enzyme; this will lead to an increased reaction rate. In order to

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improve the reaction rate of cutinase, Araújo et al. (2007) were able to modify the active site using site-directed mutagenesis. Their cutinase was more able to accommodate polymeric substrates in the active site, and showed a five-fold higher activity on PET.

7.3

Strengths and weaknesses of enzyme surface modification

The advantage of biotechnology, or more specifically enzyme technology, in the (surface) modification of textiles, is that it is possible to specifically target for a certain modification. Enzymes are biological catalysts that are involved in almost all biochemical reactions in biological systems. Enzymes increase the reaction rate of (bio)chemical reactions without being consumed themselves, just like chemical catalysts; however, enzymes are often much more effective at enhancing reaction rates than chemical catalysts. Most reactions in biological systems do occur at significant rates in the absence of enzymes. Enzyme-catalysed reaction rates are thousands to millions of times faster than those of comparable uncatalysed reactions. Enzymes perform best under ambient conditions such as atmospheric pressure, mild temperature and often a neutral pH. Enzymes are highly specific to the reaction they catalyse, not necessarily to the substrate. This enables enzymes to be used to catalyse reactions on substrates other than the ones they were originally designed for, such as synthetic polymers. In addition this results in less (unwanted) side-effects, as are often seen in chemical or chemically catalysed processes. Alkaline treatment of cellulose or polyester at elevated temperatures can also improve surface properties, but this often results in a decrease in the degree of polymerisation for cotton, or a high weight loss and decrease in bulk properties for polyester; cutinase treatment of polyester results in an even surface treatment, while the alkaline treatment results in pitting. In addition, alkaline treatment results in increased energy consumption, the necessity to neutralise the pH of the liquid and increased water consumption; these are major reasons to apply enzymes in textile processing as opposed to harsh chemicals. Enzymes have the potential to meet future requirements; this is reflected in the global consumption of enzymes in industrial/white biotechnology which increased from US$ 1.5 billion in 2000 to US$ 2.25 billion in 2007, the textile industry has a market share of approximately 10%. The weaknesses of enzyme surface modification techniques are related to the slow diffusion of enzymes compared with regular chemicals (Nierstrasz and Warmoeskerken, 2003), the limited temperature stability and the sometimes low reaction rates on synthetic materials. This low reaction rate is not necessarily a disadvantage, for targeted surface modifications it could even be an advantage. Recent developments in the field of thermostable enzymes, protein engineering and enzymes obtained via genetically modified micro-organisms (e.g. Araújo et

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al., 2007), as well as screening for new enzymes, could help to overcome the hurdle of the lower reaction rate.

7.4

Future trends

Enzymatic surface modification of textile materials is here to stay. As stated by Gübitz and Cavaco-Paulo (2008) in their review paper on surface hydrolysis and functionalisation of synthetic polymers ‘enzymes go big’. Enzymatic surface modification is not just limited to the enzymes and materials mentioned in this chapter. For example, Battistel et al. (2001), Matamá et al. (2007) and Tauber et al. (2000) reported on the enzymatic modification of acrylics using nitrilases and nitrile hydratases. New enzymes with improved activity towards synthetic, natural and biopolymers have been isolated; breakthroughs were realised via novel strategies and chemoenzymatic approaches, as well as genetic engineering to obtain improved enzymes. New insights obtained via molecular modelling will be of great importance in the design of novel enzymes, especially in the area of synthetic polymers. Novel enzyme technology for (nano-)structuring and functionalisation of textile surfaces will be the outcome. As mentioned previously, future developments will be in the field of thermostable enzymes from extremophiles, protein engineering and genetically modified enzymes. More research is needed to design effective and competitive processes that can be implementated in the textile industry. Cellulases and pectinases can be used in more conventional textile machinery, however for more advanced surface modifications and functionalities sophisticated techniques such as inkjet technology should be explored. As we have seen in this chapter, great efforts have been made to functionalise (bio)polymer materials using enzymes, and enzymes have in principle the ability to bind host- or drug-molecules to fibres, for example using XET-based chemo-enzymatic technology. In addition to enzymatic functionalisation, other biotechnological approaches have been developed. For example, cellulose binding domains (CBDs) have immense potential in the functionalisation of cellulosics (Levy and Shoseyov, 2002), e.g. cellulose-based affinity systems using functionalised CBDs, fabric care using a softening proteinfunctionalised CBD, as well as CBD-based biosensors and CBD-functionalised paper fibre which is more hydrophobic than untreated paper fibres. Another example of materials surface functionalised with enzymes is the immobilisation of enzymes on textiles. The immobilisation of enzymes on textiles to introduce functionalities such as biosensing, reactive fibres, wound healing or cleaning of delicate surfaces such as paintings, sensitive skin regions or wounds, is an existing area of research as well as a future trend. Immobilisation of enzymes allows reuse of enzymes and enables them to be used in continuous processes (e.g. in waste water treatment or downstream processing); sometimes the stability of the immobilised enzyme is also improved. Textiles with immobilised enzymes have

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the advantage of adjustable porosity, flexible construction and easy handling, as well as low costs and wide availability of the carrier material. Immobilisation of enzymes on textiles, natural as well as synthetic, is often via chemical methods (covalent bonding, cross-linking, grafting). Immobilisation of enzymes on woven silk fibres was described three decades ago (Grasset et al., 1977); silk fibroin fibres, powder and membranes were used as support for enzymes in the design of biosensors for glucose, uric acid and hydrogen peroxide (Zhang, 1998). The immobilisation of catalase on cotton fabrics – for removal of hydrogen peroxide from, for example, bleaching liquors – has been reported quite recently (Opwis et al., 2004; Wang et al., 2008). Nouaimi et al. (2001) and Nouaimi-Bachmann et al. (2007) studied the immobilisation of trypsin and co-immobilisation of trypsin, amylase and lipase on nonwoven polyester surfaces for treatment and cleaning of delicate surfaces; the immobilisation was achieved direct or via spacers. One hurdle to immobilisation is the low long-term stability caused by the low surface polarity of the PET surface. Moeschel et al. (2003) immobilised thermolysine on polyamide nonwovens, Opwis et al. (2005) developed a photochemical method to immobilise catalase on PET as well as polyamide, while Silva et al. (2007a) described a method to immobilise laccase on woven polyamide fibres. Partial hydrolysis of the polyester or polyamide surface is a strategy to increase the surface charge and to improve enzyme stability (Nouaimi et al., 2001; Silva et al., 2007a). Fairly recently, Tong et al. (2008) described a spin coating technique to coat polyester surfaces with the protease subtilisine, in order to develop a selfcleaning (textile) surface. Immobilisation of enzymes on textiles is a technology that is still in the research and development phase, but it seems to show good potential for the production of textiles with advanced properties or functionalities in the future. The development of innovative biodegradable materials will have a strong impact on the design and development of textile drug delivery systems. It is expected that functional proteins such as hydrophobins and possibly even membrane proteins will be incorporated in or on functional textile surfaces using modern biotechnology. In the near future we will see the development of more advanced functionalities, such as can be used in (enzymatic) sensors and antimicrobial products, as well as in beauty and personal care products. Enzyme technology will allow us to strengthen the bio-based economy; it is expected that enzyme technology will also have an increasing role in the production of functional textiles and thereby in our daily lives.

7.5

Acknowledgements

The author acknowledges the support of the European Commission, Marie Curie Grant, FP7-PEOPLE-2007-2-1-IEF, Grant Agreement Number PIEF-GA-2008219665, BIOTIC, Biotechnical functionalisation of (bio)polymeric textile surfaces.

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Surface modification of textiles

References

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LENTING, H.B.M., ZWIER, E. AND NIERSTRASZ, V.A. (2002), Identifying important parameters for a continuous bioscouring process. Textile Res. J., 72(9), 825–831. LENTING, H.B.M. (2004), Enzymes in textile production. In Aehle, W. (Ed.), Enzymes in Industry, 2nd edn, Section 5.2.5, Weinheim, Wiley-VCH Verlag GmbH & Co. LEVY, I. AND SHOSEYOV, O. (2002), Cellulose-binding domains. Biotechnological applications. Biotechnol. Adv., 20, 191–213. LI, Y. AND HARDIN, I.R. (1998), Enzymatic scouring of cotton-surfactants, agitation and selection of enzymes. Textile Chem. Col., 30(9), 23–29. LIEBMINGER, S., EBERL, A., SOUSA, F., HEUMANN, S., FISCHER-COLBRIE, G., CAVACOPAULO, A. AND GÜBITZ, G.M. (2007), Hydrolysis of PET and bis-(benzoyloxyethyl) terephthalate with a new polyesterase from Penicillium citrinum. Biocat. Biotrans., 25(2– 4), 171–177. LIU, Y.B., WU, G.F. AND GU, L.H. (2008), Enzymatic treatment of PET fabrics for improved hydrophilicity. AATCC Rev., 8(2), 44–48. LÓPEZ-LORENZO, M. (2004), Enzymatic fibre modification towards improved recycled paper quality. PhD Thesis, University of Twente, The Netherlands. LU, H. (2005), Insights into cotton enzymatic pretreatment. Int. Dyer, 190(3), 10–13. MATAMÁ, T., CARNEIRO, F., CAPARRÓS, C., GÜBITZ, G.M. AND CAVACO-PAULO, A. (2007), Using a nitrilase for the surface modification of acrylic fibres. Biotechnol. J., 2(3), 353– 360. MOESCHEL, K., NOUAIMI, M., STEINBRENNER, C. AND BISSWANGER, H. (2003), Immobilisation of thermolysin to polyamide non-woven materials. Biotechnol. Bioeng., 82(2), 190–199. MÜLLER, R.J., SCHRADER, H., PROFE, J., DRESLER, K. AND DECKWER, W.D. (2005), Enzymatic degradation of poly(ethylene terephthalate): rapid hydrolyse using a hydrolase from T. fusca. Macromol. Rapid Commun., 26, 1400–1405. MUZZARELLI, R.A.A. AND ILARI, P. (1994), Chitosans carrying the methoxyphenyl functions typical of lignin. Carbohydr. Polym., 23, 155–160. MUZZARELLI, R.A.A., ILARI, P., XIA, W., PINOTTI, M. AND TOMASETTI, M. (1994), Tyrosinase-mediated quinone tanning of chitinous materials. Carbohydr. Polym., 24, 295–300. NECHWATAL, A., BLOKESCH, A., NICOLAI, M., KRIEG, M., KOLBE, A., WOLF, M. AND GERHARDT, M. (2006), A contribution to the investigation of enzyme-catalysed hydrolysis of poly(ethylene terephthalate) oligimers. Macromol. Mater. Eng., 291, 1486– 1494. NIERSTRASZ, V.A. AND WARMOESKERKEN. M.M.C.G. (2003), Process engineering and industrial enzyme applications. In Cavaco-Paulo, A. and Gübitz, G.M. (Eds), Textile Processing with Enzymes, Chapter 4, Cambridge, Woodhead Publishing Ltd. NIMCHUA, T., PUNNAPAYAK, H. AND ZIMMERMANN, W. (2006), Comparison of the hydrolysis of polyethylene terephthalate fibers by a hydrolase from Fusarium oxysporum LCH I and Fusarium solani f. sp. pisi. Biotechnol. J., 2(3), 1–4. NIMCHUA, T., EVELEIGH, D.E., SANGWATANAROJ, U. AND PUNNAPAYAK, H. (2008), Screening of tropical fungi producing polyethylene terephthalate-hydrolyzing enzyme for fabric modification. J. Ind. Microb. Biotechnol., 35(8), 843–850. NOUAIMI, M., MÖSCHEL, K. AND BISSWANGER, H. (2001), Immobilisation of trypsin on polyester fleece via different spacers. Enzyme Microb. Technol., 29, 567–574. NOUAIMI-BACHMANN, M., SKILEWITCH, O., SENHAJI-DACHTLER, S. AND BISSWANGER, H. (2007), Co-immobilisation of different enzyme activities to non-woven polyester surfaces. Biotechnol. Bioeng., 96(4), 623–630.

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8 Modification of textile surfaces using nanoparticles N. VIGNESHWARAN

Central Institute for Research on Cotton Technology, India

Abstract: The application of nanotechnology on textile materials could lead to the addition of several functional properties to the base substrate. Those functional properties are of the highest importance, giving noticeable improvements in the wear comfort and care. This chapter discusses various functional properties – for example, anti-microbial, easy-care, ultravioletprotecting and flame-retardant finishes that could be achieved by the application of metal and metal oxide nanoparticles. In addition, novel applications of textile materials using nanotechnology in biological detection, decomposition of toxic gases, self-decontamination and military protection gear are discussed. Key words: antimicrobial, dispersion, easy-care, functional finishes, LotusEffect®, nanoparticles, ultraviolet protection.

8.1

Introduction

Nanotechnology deals with materials having at least one dimension in the nanometre scale, and includes nanoparticles, nanorods, nanowires, thin films and bulk materials made of nanoscale building structures (Cao, 2004). A nanometre (nm) is one billionth of a metre, or 10–9 m. Nanotechnology is not simply the continuation of miniaturization from micrometre scale down to nanometre scale but also permits entirely different and improved functionalities. For example, crystals in the nanometre scale have a low melting point, reduced lattice constants, higher surface area and catalytic activity. The band gap of semiconductors can be engineered by varying their size in the nanometre range while superparamagnetism will be observed in the case of magnetic materials. In the United States, nanotechnology has been defined as being ‘concerned with materials and systems whose structures and components exhibit novel and significantly improved physical, chemical and biological properties, phenomena and processes due to their nanoscale size’ (National Nanotechnology Initiative, 2000, www.nano.gov). Although nanotechnology is new, research in biology and colloidal science has been at a nanometre scale for centuries. The current enthusiasm for nanotechnology is driven by the availability of characterization 164

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and manipulation techniques at nanometre level and the continued shrinking of devices in the semiconductor industry, as predicted by Moore’s law. One of the attractive applications of nanotechnology in medicine is the creation of nanoscale devices, named nanorobots (Haberzettl, 2002) or simply nanobots, having the potential to act as targeted drug delivery agents. Bandgap-engineered quantum devices, such as lasers and heterojunction bipolar transistors, have been developed with unusual electronic transport and optical effects (Capasso, 1987). The invention of scanning tunnelling microscopy (STM) in the early 1980s (Binnig et al., 1982) and subsequently atomic force microscopy (AFM) (Binnig et al., 1986) have opened up new possibilities for the characterization, measurement and manipulation of nanomaterials and their bulk structures. Application of nanotechnology on textile materials could lead to the addition of several functional properties to the base substrate. For example, deposition of silver nanoparticles imparts antibacterial properties while gold nanoparticles allow the use of molecular ligands so that the presence of biological compounds in the surroundings is rapidly detected. Platinum and palladium nanoparticles impart catalytic properties such as decomposition of harmful gases or toxic industrial chemicals. More often, these nanomaterials are impregnated onto textile materials without significantly affecting their texture or comfort. An additional benefit of the use of metal nanoparticles is the presence of surface plasmons. These plasmons have strong optical extinctions that can be tuned to different colours by varying their size and shape. Silver nanoparticles can be used to create a shiny metallic yellow to dark pink colour while simultaneously imparting antibacterial properties to the textile materials. Metal oxide nanoparticles – such as TiO2, Al2O3, ZnO and MgO – possess photocatalytic and antibacterial activity and ultraviolet (UV) absorption properties. Textile materials treated with these nanoparticles have been proven to impart antimicrobial, self-decontaminating and UV-blocking functions for both military protection gear and civilian health products (Kim et al., 2002).

8.2

Nanoparticles synthesis and characterization

Owing to their extremely large surface areas, nanomaterials possess a huge surface energy and hence are thermodynamically unstable. In addition to combining individual nanostructures together to form large structures through sintering or Ostwald ripening, agglomeration is another way to reduce the overall surface energy. For practical applications, the formation of agglomerates should be prevented as they are very difficult to separate. For the fabrication and processing of nanomaterials, the following challenges must be met (Cao, 2004): (a) overcoming the huge surface energy, a result of enormous surface area or large surface to volume ratio; (b) ensuring that all nanomaterials have the desired size, uniform size

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Energy given

Bulk

Top-down

Atoms/ions Nano Energy released

Bottom-up

8.1 Schematic representation of the top-down and bottom-up approaches for the synthesis of nanomaterials.

distribution, morphology, crystallinity, chemical composition and microstructure that together result in the desired physical properties; (c) preventing nanomaterials from coarsening through either Ostwald ripening or agglomeration as time passes. The two common methods of preventing the formation of agglomerates are electrostatic stabilization (due to surface charge density) and steric stabilization (due to polymeric coating). The frequently used stabilizers are poly(vinyl pyrrolidone), polyvinyl alcohol, sodium polyacrylate, polyethyleneimine, sodium polyphosphate, starch, gelatine and proteins. There are two main approaches for the synthesis and/or fabrication of nanostructures: top-down and bottom-up. Milling and colloidal dispersion are typical examples of top-down and bottom-up methods, respectively. The bottomup approach promises a better chance of obtaining nanostructures with fewer defects and more homogeneous chemical composition, as it is mainly driven by the reduction of Gibbs free energy leading to a state closer to thermodynamic equilibrium. In contrast, the top-down approach is more likely to introduce internal stress, in addition to surface defects and contaminations (Cao, 2004). A schematic representation of the top-down and bottom-up approaches for the synthesis of nanomaterials is given in Fig. 8.1. For the synthesis of nanoparticles, apart from size, various parameters like

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8.2 Uniformly dispersed silver nanoparticles; scale bar represents 2 µm.

uniform size distribution, identical shape and chemical composition and complete dispersion contribute largely to the quality of the final product. Figure 8.2 shows the completely dispersed silver nanoparticles prepared in our laboratory. In general, nanocrystals refer to single crystalline nanoparticles and quantum dots refer to sufficiently small nanoparticles exhibiting quantum effects (in semiconductors) and surface plasmon resonance (in metals). The synthesis of metal nanoparticles, usually called colloids, can be traced back to Michael Faraday in the mid nineteenth century. Even today it is a very convenient procedure to generate gold colloids by Faraday’s method using the reduction of [AuCl4]– by citric acid. The particles formed are surrounded by an electric double layer arising from adsorbed citrate and chloride ions and by the corresponding cations. The resulting Coulomb repulsion between the particles prevents aggregation and coalescence. Figure 8.3 illustrates the situation between two particles having electric double layers. The Coulomb repulsion between the particles decays approximately exponentially with the particle distance (Bradley, 1994). Porel et al. (2007) demonstrated the synthesis of silver nanoparticles by the

168

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X–

n+

M

Mn+

X

Mn+

M

X–

X–

X–

X X–

n+

M

X–

Mn+

X–

n+ Mn+ Mn+ M

X–

–

X–

n+

Mn+

X–

–

Mn+

X– Mn+ X–

X–

X–

X–

X– –

X

Mn+

M

Mn+ X–

X–

Mn+ Mn+

X–

Mn+

X–

n+

X–

X–

X–

Mn+ Mn+

Mn+

X–

X–

X–

n+ Mn+ Mn+ M

X–

X–

X– X–

E Electrostatic repulsion r

van der Waals attraction

8.3 Electrostatic stabilization of metal colloids. Van der Waals attraction and electrostatic repulsion compete with each other. E, Coulomb repulsion; r, particle distance.

well-known ‘polyol route’ and reported the following plausible mechanism for the reduction of silver nitrate by a secondary alcohol group: R2CHOH + AgNO3 → R2CO + H2O + NO2 + Ag

[8.1]

In the case of metal oxide nanoparticles, a simple and novel aqueous route for the preparation of nanoparticles of ZnO from zinc nitrate hexahydrate without any requirement for high-temperature treatment has been reported recently (Wu et al., 2006). The possible reaction process is given as: Zn(NO3)2 • 6H2O + 2NaOH = Zn(OH)2 (gel) + 2NaNO3 + 6H2O Zn(OH)2 (gel) + 2H2O

Zn(OH)2–

[8.2]

= Zn2+ + 2OH– + 2H2O

[8.3]

= Zn(OH)2– + 2H+

[8.4]

= ZnO + H2O + 2OH–

[8.5]

Here, with the increase of the reaction temperature, the morphology of the particles seems to change from a rod-like to a short prism-like form. Similarly, the reaction

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process of sol-gel processing for the preparation of titania nanoparticles from titanium isopropoxide is: Ti(OPr)4 + 4EtOH → Ti(OEt)4 + 4PrOH

[8.6]

Hydrolysis Ti(OEt)4 or Ti(OPr)4 + H2O → Ti(OH)4 + 4PrOH or 4EtOH

[8.7]

Condensation Ti(OH)4 → TiO2 + 2H2O

[8.8]

Hydrolysis and condensation reactions occur sequentially and in parallel. Condensation results in the formation of nanoscale clusters of metal oxides, often with organic groups embedded or attached to them. Wang et al. (2005) reported a unified approach to the synthesis of a large variety of nanocrystals with different chemistries and properties and with low dispersity; these included noble metal, magnetic/dielectric, semiconducting, rare-earth fluorescent, biomedical and conducting polymer nanoparticles. This strategy was based on a general phase transfer and separation mechanism occurring at the interfaces of the liquid, solid and solution (LSS) phases present during the synthesis. Various physical and chemical techniques are being used to characterize the prepared nanomaterials. A brief account of the techniques used to analyze various parameters is given in Table 8.1.

8.3

Functional properties using nanoparticles

8.3.1

Antimicrobial finishes

‘Silver’ has been used in jewellery and for food utensils. It is a well-known fact that the growth of bacteria and microorganisms in food or water is prevented when stored in silver vessels owing to the antibacterial properties of silver, which are now scientifically recognized. Silver ions have a broad spectrum of antimicrobial activities. They are believed to get bound to protein molecules, inhibiting the cellular metabolism and leading to the termination of the growth of microorganisms. An additional benefit to the use of metal nanoparticles is the presence of surface plasmons. These plasmons have strong optical extinctions that can be tuned to different colours by varying their size and shape. Silver nanoparticles can be used to create a shiny metallic yellow to dark pink colour while simultaneously imparting antibacterial properties to the textile materials. Scanning and transmission electron microscopy were used to study the biocidal action of this nanoscale material (Sondi and Salopek-Sondi, 2004). The results confirmed that the treated Escherichia coli cells were damaged, showing the formation of ‘pits’ in the cell wall of the bacteria, while the silver nanoparticles

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Table 8.1 Techniques for characterization of nanoparticles Characterization techniques Structural characterization X-ray diffraction (XRD) Small angle X-ray scattering Scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) Transmission electron microscopy (TEM) with EDX Scanning probe microscopy (SPM) Gas absorption Chemical characterization Optical spectroscopy (UV and infrared absorption, fluorescence and Raman effect characteristics) Electron spectroscopy Ionic spectrometry

Parameters Crystal structures of nanoparticles and size of crystallites Size of nanoparticles or their surface area per unit volume Topographical information of nanomaterials and their chemical composition Particle size and chemical composition Three-dimensional real-space images Surface area, particle size and porous structures Bonding and chemical nature of the nanomaterials

Chemical composition analysis Thin-film characterization and elemental analysis

Physical characterization Melting point apparatus and XRD Melting point and lattice constants Atomic force microscopy (AFM) Mechanical properties and TEM Optical spectroscopy Surface plasmon resonance and quantum size effects SPM Electrical and magnetic properties

were found to accumulate in the bacterial membrane. A membrane with such morphology exhibits a significant increase in permeability, resulting in death of the cell. On account of its non-toxic nature, nano-silver is biocompatible and can be used effectively to reduce bacterial counts on nonwoven materials such as air and water filters, medical clothing and textile woven fabrics that come into direct contact with human skin (Tiller et al., 2001). In our earlier report (Vigneshwaran et al., 2007), we reported on a novel, one-pot synthetic route for the preparation of silver nanoparticles, reduced and stabilized by starch on the surface of cotton fabrics. Thus-formed nanoparticles impart colour to the fabrics owing to the surface plasmon resonance. Figure 8.4 shows a scanning electron micrograph of silver nanoparticles deposited on the surface of cotton fabrics. It was also shown that

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8.4 Scanning electron micrograph of cotton fibre impregnated with silver nanoparticles.

these silver nanoparticle-impregnated cotton fabrics showed excellent antibacterial activity against Staphylococcus aureus and bacteriostasis activity against Klebsiella pneumoniae. Another research group from Hanyang University, Korea (Lee and Jeong, 2005) – in their work on the padding of colloidal silver solution onto textile fabrics made from cotton, polyester, cotton/polyester and cotton/spandex blended fabrics – have also reported efficient antibacterial activity against both Staphylococcus aureus and Klebsiella pneumoniae with good laundering durability (Lee et al., 2003). Jiang et al. (2006) applied a nanolayer of silver coating onto cotton and polyester fabrics by chemical silver plating and showed that these fabrics had improved properties, including antibacterial, ultraviolet (UV) light absorption, antistatic and light-fastness properties. These special properties are due to the fact that basic silver particles have higher shielding properties and better conductivity than the original fabrics.

8.3.2

Ultraviolet-protection finishes

Prolonged and repeated exposure to UV radiation from sunlight has been identified as the cause of an increase in the incidence of skin cancer in humans. Limiting the skin’s exposure to sunlight, especially during the hours of maximum intensity, is the best way to reduce risk. For a person who must work outdoors this is not feasible, well-designed clothing made from UV-blocking textiles is the best alternative. The various processes undergone by the incident UV light on a fabric material are represented in Fig. 8.5. The transmitted and scattered light will be the

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8.5 UV-transmittance characteristics of a textile material.

Padding

Fabric Drying at 100 °C

Curing at 140 °C

Nano-ZnO + binder

8.6 Antibacterial finishing of cotton fabrics with nano-ZnO.

focus here as it is this light that is actually responsible for sunburn. ZnO nanoparticles score better than other nanoparticles in terms of cost-effectiveness, whiteness and UV-blocking properties. The UV-blocking properties of a fabric are enhanced when a dye, pigment, delustrant or UV-absorbant finish is present that absorbs UV radiation and blocks its transmission through the fabric to the skin (Hustvedt and Crews, 2005). Metal oxides such as ZnO are more stable as UV blockers when compared with organic UV-blocking agents. Hence, the nano-form of ZnO will really enhance UV-blocking properties due to the increased surface area and intense absorption in the UV region. Our research (Vigneshwaran et al., 2006) has indicated excellent antibacterial activity against two representative bacteria, Staphylococcus aureus and Klebsiella pneumoniae, and promising protection against UV radiation for the nano-ZnO-impregnated cotton textiles. Figure 8.6 shows the steps in the process of coating cotton fabrics with nano-ZnO in our

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173

8.7 Scanning electron micrograph of nano-ZnO (embedded inside starch granule) coated on cotton fibres.

laboratory and Fig. 8.7 shows a scanning electron micrograph of nano-ZnO (embedded inside starch granule)-coated cotton fibres. The ultraviolet protection factor (UPF) was calculated using the following equation (AATCC Test Method 183–2006 (AATCC, 2006)): 400

Σ Eλ × Sλ × ∆λ λ=280 UPF = ––––––––––––––––––– 400 Σ Eλ × Sλ × Tλ × ∆λ

[8.9]

λ=280

where Eλ is the relative erythermal spectral effectiveness, Sλ is the solar spectral irradiance, Tλ is the average spectral transmission of the specimen and ∆λ is the measured wavelength interval (nm). The UPF equation weighs the UV-B radiation more heavily than UV-A. The UPF ratings are as follows: • • • •

50+, maximum achievable; 40–50, excellent protection; 25–39, very good protection; 15–24, good protection.

The nano-ZnO-impregnated fabrics were found to retain more than 80% efficiency of both antibacterial and UV-protection functions even after 25 hand-wash cycles. The wash fastness is generally studied by a standard test, AATCC Test Method 61–2006, where a single wash cycle corresponds to five typical careful

174

Surface modification of textiles

hand launderings. The loss and changes resulting from the detergent solution and the abrasive action of five typical hand launderings are roughly approximated by this 45 min test. The abrasive action is a result of the frictional effects of fabric against the canister, the low liquor ratio and the impact of the steel balls on the fabric. In yet another study (Vigneshwaran et al., 2007), nano-silver-coated fabrics exhibited better protection against UV radiation owing to the nano-silver absorption in the near-UV region. Gorensek and Recelj (2007) observed similar increases in UV absorption when they functionalized cotton fabric with commercial nano-silver.

8.3.3

Easy-care finishes

If the critical surface tension of a solid fabric is greater than or equal to the surface tension of a liquid, the liquid will wet the fabric. If the critical surface tension of the solid is less than surface tension of the liquid, the fabric will repel the liquid. In the case of solids ‘critical surface tension’ is used instead of ‘surface tension’. Thus, water repellency can be attained when the critical surface tension of the solid is smaller than surface tension of the liquid. For example, when a drop of water is dripped on a cotton fabric, it has been experimentally determined that the surface tension of water and the critical surface tension of cotton are, respectively, 72 dyne/cm and 200 dyne/cm, and, therefore, water readily wets the cotton fibre. However, once the cotton is treated with a fluorocarbon the water-repellency relation between them changes. The critical surface tension of water-repellentfinished cotton is less than the surface tension of water. Fluorocarbons are organic compounds consisting perfluorinated carbon chain. They tend to decrease the surface tension of the substrate (Zabicky, 2006). The self-cleaning of super-hydrophobic micro- to nanostructured surfaces was observed to be a property of some plants – one of which is nasturtiums (Tropaeolum sp.). In 1975, the researcher Wilhelm Barthlott, then at the University of Heidelberg, elucidated this phenomenon. Only at the beginning of the 1990s was it possible for W. Barthlott, and his collaborator Christoph Neinhuis (later Zdenek Cerman and others) to harness this physico-chemical phenomenon technically. Barthlott coined the trade name Lotus-Effect® for the patented self-cleaning super-hydrophobic micro- to nanostructured products and copyrighted it in 1997 (http://www.lotuseffekt.de/en/faq/index.php). Over millions of years of evolution nature has developed surfaces – the ideal example being the leaves of the lotus (Nelumbo nucifera) – which, through a complicated micro- to nano-scale surface structure, repel dirt (see Fig. 8.8). The surfaces concerned are those that, owing to their chemistry, are water repellent (hydrophobic). These particular properties, however, are not only determined by the chemistry but also, in particular, the micro- to nanostructure: in the ideal case this is composed of a bulky structure (5–10 µm in diameter) and a fine structure (10 nm to 5 µm) layered on top. Adhesion of particles to these surfaces is minimal because they touch only the tips of the surface

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8.8 Scanning electron micrograph of lotus leaf.

Nano-coated textile surface

8.9 Lotus-Effect® removing the dirt particles from super-hydrophobic surfaces.

structure. The particles are washed away by water droplets, which roll off. As a result, the surfaces stay dry even during a heavy shower. Furthermore, the droplets pick up small particles of dirt as they roll, and so the leaves of the lotus plant keep clean even during light rain. The technical expression of these basic ideas is patented as the Lotus-Effect, and the surface acquired the aspect shown in Fig. 8.9; Fig. 8.10 shows a scanning electron micrograph of a super-hydrophobic textile material.

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8.10 Scanning electron micrograph of super-hydrophobic textile material created using nanotechnology.

Nanocrystalline TiO2 coatings have received much attention as photocatalysts in practical applications such as environmental purification, deodorization, sterilization, antifouling and self-cleaning glass, owing to their high oxidizing ability, non-toxicity, long-term stability and low cost. Among the different crystalline phases of titania, anatase is reported to have the best performance. Daoud and Xin (2004) successfully grew anatase nanocrystallites on cotton fabrics and these fabrics could be made into self-cleaning clothes that tackle dirt, environmental pollutants and harmful microorganisms. In addition, they have reported (Qi et al., 2006) on the application of a transparent thin layer of nanocrystalline titania coating on cotton textiles using a dip-pad–dry-cure process. These titania-coated cotton textiles possess significant photocatalytic self-cleaning properties, such as bactericidal activity, colourant stain decomposition and degradation of red wine and coffee stains. Figure 8.11 shows a schematic representation of the photocatalytic behaviour of titania nanoparticles; organic compounds are degraded on exposure to UV light. Another study (Bozzi et al., 2005) reported on radiofrequency plasma (RFplasma), microwave plasma (MW-plasma) and vacuum-UV light irradiation as pretreatments for synthetic textile surfaces, allowing the loading of TiO2 by wet chemical techniques in the form of transparent coatings constituted of nanoparticles of diverse sizes. These loaded textiles show a significant photo-oxidative activity under visible light in air under mild conditions, which discolours and mineralizes persistent pigment stains contained in wine and coffee. The mineralization of stains on the textile loaded with TiO2 was monitored quantitatively to assess the appropriate surface pretreatment in conjunction with the most suitable deposition

Modification of textile surfaces using nanoparticles

UV

177

–

lig

O2

ht

TiO2 O2 Electron

e– 3.3 eV

Hole

+ Organic compound CO2 + H2O

h+

H2O

OH

–

8.11 Schematic representation of the photocatalytic behaviour of titania nanoparticles.

method for TiO2 colloids, powders, or a combination of both; their photocatalytic activity allowed, in kinetically acceptable times, the almost complete discoloration of coffee and wine stains. The observed discoloration of coloured stains seems to involve visible light sensitization of the stain pigment on the TiO2-loaded textile. The size of the particles obtained from colloidal precursors of TiO2 varied between 5 and 25 nm. The rate of super-hydrophobicity is measured by determining the so-called ‘repellent power’, which was first introduced by Dr Keller, BASF, Germany, via the determination of the dynamic roll-off angle. The static dynamic contact angle used for the characterization of even surfaces such as foils is applicable to textiles only in special cases. Recently, a water-repellent nanocoating – consisting of TiO2 nanoparticles together with a hydrophobic fluoromethylic copolymer coating – was demonstrated on cement plate and cotton fabrics. Seven parameters – including the type of nanoparticle, solid ratio, dispersion time, fluoro-binder ratio, distance between nozzle and substrate, spray direction and layer number – were considered according to the construction analysis. The Taguchi method and the analysis of variance indicated that solid ratio had an important effect on the water repellency of the surface, i.e. it showed the highest contribution percentage of 48.2% (Lin et al., 2006). In order to prove the super-hydrophobic and self-cleaning effects of textile products, ITV Denkendorf (Germany) issues the quality mark ‘self-cleaning – inspired by nature’; this quality mark is issued based on the results of conventional testing methods and scanning electron microscopy (SEM) examination.

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8.3.4

Surface modification of textiles

Other functional finishes

New-generation medical textiles are an important and growing field, requiring functional properties such as bacteriostatic, antiviral, fungistatic, non-toxic, high absorbance, non-allergic, breathable, haemostatic and biocompatible properties. Therefore, in addition to metal and metal oxide nanomaterials, nanoscale biological materials such as enzymes and drugs are necessary to add specific functionality to medical textiles (Petrulyte, 2008). Specialized nanomaterials functionalized with ligands can be introduced on the surface of cotton textiles with the aim of absorbing odours, providing strong and durable antibacterial properties, easing pain and relieving irritation. In addition, such value-added textiles could be of immense use in tissue engineering, drug delivery and protective clothing. Nyacol® Nano-technologies Inc. has been the world’s leading supplier of colloidal antimony pentoxide which is used for flame-retardant finishes on textiles. It supplies colloidal antimony pentoxide as a fine particle dispersion for use as a flame-retardant synergist with halogenated flame retardants. The ratio of halogen to antimony ranges from 5:1 to 2:1. Green-shield®, a nanotech firm from Taiwan, has created an underwear that aims to eliminate odour. The underwear textile material releases undetectable negative ions and infrared rays that destroy the odour-causing bacteria; also, the far-infrared rays are absorbed by cells causing all the individual atoms to vibrate at a higher frequency, which speeds up the

8.12 Design and electricity-generating mechanism of the fibre-based nanogenerator driven by a low-frequency, external pulling force. (a) Schematic of the experimental set-up of the fibre-based nanogenerator. (b) An optical micrograph of a pair of entangled fibres, one of which is coated with Au (in darker contrast). (c) SEM image at the ‘teeth-to-teeth’ interface of two fibres covered by nanowires (NWs), with the top one coated with Au. The Au-coated nanowires at the top serve as the conductive ‘tips’ that deflect/bend the nanowires at the bottom. (d) Schematic illustration of the teeth-to-teeth contact between the two fibres covered by nanowires. (e) The piezoelectric potential created across nanowires I and II under the pulling of the top fibre by an external force. The side with positive piezoelectric potential does not allow the flow of current owing to the existence of a reverse-biased Schottky barrier. Once the nanowire is pushed to bend far enough to reach the other Au-coated nanowire, electrons in the external circuit will be driven to flow through the uncoated nanowire due to the forward-biased Schottky barrier at the interface. (f) When the top fibre is further pulled, the Au-coated nanowires may scrub across the uncoated nanowires. Once the two types of nanowires are in final contact, at the last moment, the interface is a forward-biased Schottky, resulting in further output of electric current, as indicated by arrowheads. The output current is the sum of all the contributions from all of the nanowires, while the output voltage is determined by one nanowire.

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179

(a) Fixed end

Fibre covered by ZnO NWs

Fibre covered by ZnO NWs and coated with Au

Stretching direction

Fixed end

Pulling string

Fixed end

Spring

+

–

–

(b)

100 µm

(c)

(d) Fibre base

Au-coated ZnO NWs Au-coated ZnO NWs

ZnO NWs


ZnO NWs Fibre base

1 µm

(e)

(f)

Stretching direction

V+

Stretching direction

V+

V+ V–

V–

V–







–

–

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Surface modification of textiles

metabolism and the elimination of wastes. This nanofinish could eliminate up to 99.99% of bacteria, 90% of odour and 75% of sticky moisture within the cloth as well as contributing to the overall health of the wearer. Recently, a simple, low-cost approach has been reported (Qin et al., 2008) that converts low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibres. By entangling two fibres and brushing the nanowires rooted on them with respect to each other, mechanical energy is converted into electricity owing to a coupled piezoelectric– semiconductor process. This work establishes a methodology for scavenging light-wind energy and body-movement energy using fabrics. Figure 8.12 shows the design and electricity-generating mechanism of the fibre-based nanogenerator driven by a low-frequency, external pulling force. Apart from silver nanofinishing for an antimicrobial finish, nanoparticles consisting of a drug either surrounded by a synthetic, polymer shell or contained within a synthetic, three-dimensional polymer matrix, at scales ranging from micrometric to nanometric, can be used for drug delivery in medical textiles. Encapsulation can be achieved by several methods, e.g. interfacial polymerization, microemulsion polymerization, precipitation polymerization and diffusion. In general, the drug is brought into contact with a set of monomers, oligomers or polymers. These assemble around the payload; polymerization will give the final particles. An alternative method is to prepare the nanoparticle in the absence of the drugs, which are then absorbed by the nanoparticles following afterwards (Soane et al., 2001). Another MIT (Massachusetts Institute of Technology) investigation is a cloth linking nanoparticles of gold in solution with strands of DNA coded to change colour when exposed to the DNA of biological agents, so your shirt could detect low doses of chemicals in the air, for instance, or your dressing gown could diagnose viruses like flu or SARS (severe acute respiratory syndrome).

8.4

Commercialization of nanofinishing in textiles

The use of nanotechnology-based finishes to enhance the performance of textiles made from natural fibres (including cotton, wool and silk) and also from synthetic fibres (such as polyester and nylon) is growing fast. The Lotus Effect has been emulated in textiles using nanotechnology. In the NANO-CARE technology of Nano-Tex, LLC, USA, the textile is embedded with billions of nanowhiskers of 10 nm in length. These nanowhiskers cover the textile, making it so dense that liquids can hardly penetrate. In the NanoSphere technology of Schoeller Textiles AG, Switzerland, a special three-dimensional surface structure of nanospheres 1–100 nm in size is created in textiles, limiting the available contact surface for dirt particles; the NanoSphere finishing process renders fabric water and dirt repellant and anti-adhesive. Major companies involved in the nanofinishing of textiles include:

Modification of textile surfaces using nanoparticles (a) (b) (c) (d)

181

Nano-Tex, LLC, USA; Texcote Technology Ltd, Sweden; Schoeller Textiles AG, Switzerland; Beijing Zhong-Shang Centennial Nano-Tech Co. Ltd, China.

To date, the world’s 20 largest textile mills have acquired licensing for Nano-Tex technology. Products provided by Nano-Tex are: • NANO-CARE for stain resistance, wrinkle resistance and liquid repellency on cotton; • NANO-PELTM for fabric that breathes, yet remains liquid- and stain-repellent; • NANO-DRY for enhanced fabrics able to move perspiration away from the body, while drying quickly; • NANO-TOUCHTM, which gives man-made fabrics the feel and comfort of natural fabrics; • Another product designed to capture body odor, i.e. NANO-FRESHTM. Nanotechnology, although still very much in its infancy, is already proving to be a useful tool for improving the performance of textiles. With increased performance comes added value and additional revenue. One company to realize this has been the Burlington Industries subsidiary, Nano-Tex. Branded as one of the ‘coolest’ products in 2003 by Time Magazine, Nano-Tex is providing clothing manufacturers such as Levi’s, Eddie Bauer, Gap and Old Navy the means to make their products more durable, more water and oil repellent, and more stain resistant, while also reducing the need for washing – all without altering the feel of the fabric. Nano-Tex’s chemical formulation and application technology, which is easily adopted by existing textile mills, changes the fabric itself on a molecular level, embedding it with tiny, floppy, hair-like fibres that themselves are attached to a common spine. The ‘nanowhiskers’ in the chemical mix prevent stains from soaking into clothing. Nano-Tex is said to have plans to expand its product range to include stain-proof mattresses, boat covers and hotel bedding. Looking at the previously stated definition of nanotechnology it could be argued that Nano-Tex’s technology is not really nanotechnology but improved chemistry; however the company is realizing a profit while other ‘proper’ nanotechnology companies are still waiting and dreaming. Toray Industries, Inc. developed a nanoscale processing technology that allows the molecular arrangement and molecular self-assembly that are necessary to bring about further advanced functionalities in textile processing. This technology is named ‘NanoMATRIX’, in which a functional nanoscale material coating is applied to each of the monofilaments that forms a woven or knitted fabric. Application of this technology is expected to lead to the development of new functionalities, the creation of complex functionalities, remarkable improvements in the existing functions (quality, durability, etc.) and expansion of usage in terms of materials/applications without losing the fabric’s texture.

182

8.5

Surface modification of textiles

Strengths and weaknesses of nanotechnology for surface modification

Microbes are developing an important role in manufacturing systems owing to their ability to synthesize and accumulate metal nanoparticles; they have been used experimentally to remove metal nanoparticles from effluent. The silver nanoparticles in the effluent from a cotton nanofinishing process could be removed efficiently by the use of Chromobacterium violaceum (Duran et al., 2007), which is able to absorb, metabolize or store metal ions, and thus environmental damage will be avoided. With the onset of the Industrial Revolution, concerns relating to public health and safety and the environment have resulted in increasingly stringent environmental regulations (Theodore, 2006). Hence, there is a need for the development of fully-fledged norms and regulations for the use of nanomaterials and their disposal in the environment. In the case of textiles, it will be of great concern that both the finished product and the effluent will comply with these regulations. Since textile products have the closest interaction with our body, they must be evaluated for their toxicology and tolerance levels.

8.6

Future trends

Nanotechnology overcomes the limitations of conventional methods when imparting various functional properties to textile materials. Those functional properties are of the highest importance, and give noticeable improvements in the wear comfort and care. Despite great research effort into the nanofinishing of textiles, their commercial exploitation has only just begun. The main emphasis in the application of nanotechnologies to textiles will be to (Holme, 2005): • improve the properties and performance of existing materials; • develop smart and intelligent textiles with novel functions; • greatly increase the use of fibres in technical textiles, biomedical textiles and healthcare applications; • open up new opportunities for fibres as sensors.

8.7

References

AATCC (2006) Test Methods, 183-2006 and 61-2006, in AATCC Technical Manual, The American Association of Textile Chemists and Colorists, IHS, Englewood, Colorado. BINNIG G, QUATE C F AND GERBER C H (1986), ‘Atomic force microscope’, Phys Rev Lett, 56, 930–933. BINNIG G, ROHRER H, GERBER C AND WEIBEL E (1982), ‘Surface studies by scanning tunneling microscopy’, Phys Rev Lett, 49, 57–61. BOZZI A, YURANOVA T AND KIWI J (2005), ‘Self-cleaning of wool-polyamide and polyester textiles by TiO2-rutile modification under daylight irradiation at ambient temperature’, J Photochem Photobiol A: Chem, 172, 27–34. BRADLEY J S (1994), in Schmid G (Ed.), Clusters and Colloids: From Theory to Applications, VCH, Weinheim, p. 459.

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CAO G (2004), Nanostructures and Nanomaterials – Synthesis, Properties and Applications, Imperial College Press, London. CAPASSO F (1987), ‘Band-gap engineering: from physics and materials to new semiconductor devices’, Science, 235, 172–176. DAOUD W A AND XIN J H (2004), ‘Synthesis of single phase anatase nanocrystalline at near room temperature’, J Am Ceram Soc, 87, 953–955. DURAN N, MARCATO P D, SOUZA G I H D, ALVES O L AND ESPOSITO E (2007), ‘Antibacterial effect of silver nanoparticles produced by fungal process on textile fabrics and their effluent treatment’, J Biomed Nanotechnol, 3, 203–208. GORENSEK M AND RECELJ P (2007), ‘Nanosilver functionalized cotton fabric’, Textile Res J, 77(3), 138–141. HABERZETTL C A (2002), ‘Nanomedicine: destination or journey’, Nanotechnology, 13, R9– R13. HOLME I (2005), ‘Nanotechnologies for textiles, clothing, and footwear’, Textiles Mag, 1, 7– 11. HUSTVEDT G AND CREWS P C (2005), ‘The ultraviolet protection factor of naturally pigmented cotton’, J Cotton Sci, 9, 47–55. JIANG S Q, NEWTON E, YUEN C W M AND KAN C W (2006), ‘Chemical silver plating on cotton and polyester fabrics and its application on fabric design’, Textile Res J, 76(1), 57– 65. KIM Y K, RICE J M, LANGLEY K D, LEWIS A F, SEYAM A, PAWAR S AND KUMBHANI M (2002), National Textile Center Annual Report MOOD08, http://www.ntcresearch.org/ pdf-rpts/AnRp08/F08-MD01-A8.pdf. LEE H J AND JEONG S H (2005), ‘Bacteriostasis and skin innoxiousness of nanosize silver colloids on textile fabrics’, Textile Res J, 75(7), 551–556. LEE H J, YEO S Y AND JEONG S H (2003), ‘Antibacterial effect of nanosized silver colloidal solution on textile fabrics’, J Mater Sci, 38, 2199–2204. LIN T S, WU C F AND HSIEH C T (2006), ‘Enhancement of water-repellent performance on functional coating by using the Taguchi method’, Surface and Coat Technol, 200(18–19), 5253–5258. PETRULYTE S (2008), ‘Advanced textile materials and biopolymers in wound management’, Danish Med Bull, 55(1), 72–77. POREL S, VENKATRAM N, RAO D N AND RADHAKRISHNAN T P (2007), ‘In situ synthesis of metal nanoparticles in polymer matrix and their optical limiting applications’, J Nanosci Nanotechnol, 7, 1887–1892. QI K, DAOUD W A, XIN J H, MAK C L, TANG W AND CHEUNG W P (2006), ‘Self-cleaning cotton’, J Mater Chem, 16, 4567–4574. QIN Y, WANG X AND WANG Z L (2008), ‘Microfibre–nanowire hybrid structure for energy scavenging’, Nature, 451, 809–813. SOANE D S, OSFORD D A, WARE W JR, LINFORD M R, GREEN E AND LAU R (2001) Worldwide Patent WO 0106054 A1. SONDI I AND SALOPEK-SONDI B (2004), ‘Silver nanoparticles as antimicrobial agent: a case study on E. coli as a model for Gram-negative bacteria’, J Colloid Interface Sci, 275, 177– 182. THEODORE L (2006), Environmental regulations, in Nanotechnology: Basic Calculations for Engineers and Scientists, Chapter 21, Wiley-Interscience, USA, p. 333. TILLER J C, LIAO C J, LEWIS K AND KLIBANOV A M (2001), ‘Designing surfaces that kill bacteria on contact’, Proc Natl Acad Sci USA, 98, 5981–5985.

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VIGNESHWARAN N, KATHE A A, VARADARAJAN P V, NACHANE R P AND BALASUBRAMANYA R H (2007), ‘Functional finishing of cotton fabrics using silver nanoparticles’, J Nanosci Nanotechnol, 7, 1–5. VIGNESHWARAN N, SAMPATH KUMAR T S, KATHE A A, VARADARAJAN P V, VIRENDRA PRASAD (2006), ‘Functional finishing of cotton fabrics using zinc oxide–soluble starch nanocomposites’, Nanotechnology, 17, 5087–5095. WANG X, ZHUANG J, PENG Q AND LI Y (2005), ‘A general strategy for nanocrystal synthesis’, Nature, 437, 121–124. WU C, QIAO X, CHEN J, WANG H, TAN F AND LI S (2006), ‘A novel chemical route to prepare ZnO nanoparticles’, Mater Lett, 60, 1828–1832. ZABICKY J (2006), ‘Textiles stain repellency and self cleaning’, Advanced Materials Engineering, Zvi Reinstein, http://portal.jce.ac.il/courses/nano/Nanomaterials%20Projects/ Self-cleaning%20textiles.pdf.

9 Modification of textile surfaces using the sol-gel technique T. TEXTOR

Deutsches Textilforschungszentrum Nord-West e.V., Germany

Abstract: Many researchers regard the sol-gel technique as the most important development in material science in the last decade. The sol-gel technique offers far-reaching possibilities for surface modification of a variety of different materials, not only textile materials. The general approach allows one not only to functionalize textile materials with conventional application technologies under moderate conditions, but also to combine various functionalities in a single material. The following section will introduce the basic principles of the sol-gel technique and also explain some general aspects regarding the textile applications discussed in this chapter. Some examples will be given for certain modifications of textiles, for example improved repellence, ultraviolet absorption or wear resistance, the application of barrier layers or the creation of self-cleaning surfaces that demonstrate the photocatalytic effect. Key words: nanosols, nanotechnology, inorganic–organic hybrid polymer, surface modification.

9.1

Introduction: the principles of the sol-gel technique

In parallel with the triumphant success that nanotechnology has achieved within the last decade, the sol-gel technique has become an important tool for producing nanoparticles or for the preparation and application of thin layers or coatings based on either inorganic materials or inorganic–organic hybrids. The sol-gel technique is of particular importance for textile materials since it promises far-reaching possibilities for developing improved and new products, especially with regard to the growing market of technical textiles. The sol-gel technique allows one to tailor certain properties and to combine different properties in a single coating step. The principles of the sol-gel process can be explained by means of probably the best investigated example, the silica sol. The general process can basically be divided into three main steps: hydrolyzation, application and curing. The first step is the hydrolyzation of the precursors, for example tetraethoxysilane (TEOS). The hydrolyzation is mostly carried out in a solvent such as an alcohol, a ketone or mixtures of water and organic solvents. The hydrolyzation will lead to the 185

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Surface modification of textiles

formation of the corresponding silanols, forming silicic acids. Silicic acid is only stable in comparably low concentrations (usually <2 × 10–3 mol/L in water). At higher concentrations the silicic acid tends to undergo condensation reactions, first forming various polysilicic acids. Depending on the reaction conditions (pH value, concentration, solvent, temperature, presence of salts or other additives) ongoing condensation reactions lead to the formation of more or less discrete silica nanoparticles. The resulting dispersions – also called nanosols – are stable over a wide time scale from less than a minute to years. As an example, by evaporation of the solvents, increase of the temperature, changing the pH value or the addition of salts, condensation reactions as well as agglomeration reactions are speeded up, resulting in an accelerated gelation process. In the initial period of gelation, socalled lyogels are formed (three-dimensional gel networks, in which the pores are filled with the solvent): the third step, curing or drying, will result in xerogels after the solvent is completely removed. These xerogels are amorphous silica networks – since the sizes of the building blocks are in the range of nanometers, the amorphous networks will appear as homogeneous, transparent layers. Further tempering of the xerogel at a temperature higher than 400 ºC would be necessary to convert the amorphous material into crystalline silica oxide. Since textile materials cannot withstand these conditions (if we disregard inorganic fiber materials) the sol-gel process stops after formation of the xerogel. Figure 9.1 summarizes the above described three steps of the sol-gel process. The silica sol has been presented as an example only because it is probably the most investigated sol. Nowadays sol-gel science allows the preparation and stabilization of a huge number of metal oxide sols (also based on mixed oxides). Table 9.1 shows an overview of the most prominent examples. The concept of the sol-gel technique presents far reaching possibilities for the modification of the inorganic metal oxide materials introduced above. This concept allows, for example, a readily prepared nanosol to be mixed with an apparently unlimited number of compounds – polymers, pigments, drugs, fragrances, dyestuffs or nanopowders, to name but a few. These compounds must be miscible with the sol, they may be dissolved into the sol or they may be dispersed. Possibly the only restriction is that the compound must not initiate the gelation process, which would impair further processing of the sol. After the application and formation of the xerogel these compounds will be at least

H2O/catalyst

Si(OR)4 Precursor

(SiO2)x Sol

Application

(SiO2)y Lyogel

Curing

(SiO2)y Xerogel

9.1 Schematic summarizing the different steps of a surface modification employing the sol-gel process (the example shown is for a silica sol based on a silicon alkoxide).

Modification of textile surfaces using the sol-gel technique

187

Table 9.1 Selection of metal oxides that can be prepared by the sol-gel technique and possible applications Metal oxide

Possible application/properties

Reference

TiO2

UV protection

TiO2 (anatase)

Photocatalytic effect

ZnO

UV protection

BaSO4 SnO2 Fe3O4

Shielding of X-/α-rays Conducting (antistatic effect) Magnetic, superparamagnetic effects

Xin et al. (2004); Xin and Daoud (2005); Xu et al. (2005a) Daoud and Xin (2004a. 2004b); Iwashita et al. (2000); Liuxue et al. (2008); Textor et al. (2003, 2007) Huang et al. (2006); Li et al. (2007); Textor et al. (2003) Qu et al. (2002) Wu et al. (2004) Thomas et al. (2006); Xu et al. (2005b); Zorjanovic et al. (2004)

physically embedded into the pores of the resulting network, depending on the functionality the additives might also be bound to the matrix if the compounds are able to form chemical bonds during the formation of the xerogel. In the above-mentioned example the nanosols act more or less as a simple binder system. Appropriate choice of precursor makes it possible to change the properties of the matrix/network itself, for example by introducing precursors such as organically modified silanes, either into the initial mixture (before the hydrolyzation) or to the readily prepared sols; new materials are available that are also called inorganic–organic hybrid polymers (some well-known names are: Ormocer®, Organosil, Nanomer®). These organically modified silanes can undergo the same sol-gel reactions described above (hydrolyzation/condensation) and form stable sols. In combination with the precursors used for the preparation of inorganic nanosols, the mixtures allow preparation of hybrid systems. By introducing the organically modified silanes with functional organic groups into the matrices one introduces an interface that allows, for example, the combination with compounds known from conventional polymer chemistry; it allows one to bind certain molecules/materials covalently to the resulting matrices. A selection of corresponding commercially available silanes is given in Fig. 9.2. The networks resulting from sols based on cross-linkable organically modified silanes show inorganic as well as organic domains (see Fig. 9.3); since these materials combine certain properties of ceramic materials and organic polymers they are also described as organically modified ceramics (which is sometimes misleading since a ceramic material has a certain crystallinity, while the networks described here are fundamentally amorphous). As with the inorganic nanosols, the organically modified hybrid polymer sols can also be combined with many different materials.

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9.2 Selected organically modified alkoxysilanes that are commercially available.

X

+ H2O – CH3OH ∆T (catalyst)

= Matrix elements, not particularly specified

9.3 Sol-gel-derived network of an organic–inorganic hybrid polymer exhibiting organic (here polyethylene oxide groups) as well inorganic domains (here SiOx groups). X, unknown number of these molecules.

Modification of textile surfaces using the sol-gel technique

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Organic domain (cross-linking) Network modifier Nanoparticle

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9.4 Basic structural elements that can be introduced to modify the properties of an inorganic–organic hybrid polymer network.

The intention of this very brief introduction to the basics of the sol-gel technique is to make the reader conscious of the really tremendous possibilities for modifying a basic sol-gel system and therefore for modifing a material’s properties. The combination of different sols and/or additives will allow one to achieve, and also combine a huge number of properties – some authors speak of tailoring the properties of the resulting materials. Figure 9.4 summarizes the basic structural elements that can determine the properties of an inorganic–organic hybrid polymer derived using the sol-gel technique. Nanoparticles introduced into the networks might, for example, increase the hardness of the materials (Al2O3), or increase their density, or may result in excellent ultraviolet (UV) absorption (ZnO, TiO2). By the choice of organic cross-linkers the flexibility of the hybrid polymers can be improved or the share of organic material can be increased if necessary. Organic network modifiers might be added to decrease the network density or they can, for example, be used to adjust the surface energy of the resulting material. Moreover the networks can immobilize (temporarily as well as permanently) a huge number of materials as mentioned above. Since the basic sol-gel approach is compatible with the finishing techniques conventionally used in textile industries, the transfer of this technique to textile industries seems to be reasonable and has already been successfully achieved for a limited number of products. Moreover, it offers a tremendous number of

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Shielding of X-/α-rays Magnetic Flame resistance

Enzyme immobilization Superparamagnetic UV protection

Antiwrinkle

Heat resistance

Photocatalytic effect

Hydrophobic Antistatic properties

Stab resistance

Improved abrasion stability Dyeing

Photochromic

Oleophobic Self-cleaning

Controlled release

9.5 Incomplete list of keywords found in literature when the terms ‘sol-gel’ and ‘textile’ are used as search terms.

promising approaches. Figure 9.5 is a collection of keywords to give an, admittedly incomplete, overview of the functionalization effects that have already been described in the literature. The following sections will therefore show some general aspects of the coating or modification of textiles with sol-gel-derived materials, as well as describing in more detail selected properties that can be achieved by applying appropriate sols.

9.2

General aspects of textile finishing using (nano-)sols

As mentioned above, (nano-)sols can be applied by conventional coatings techniques used in the textile industry. The application can be carried out by a simple dipping process, spraying, nip-padding or padding. One of the enormous drawbacks of the sol-gel technique from an industrial point of view is that most sols are not aqueous. The main reason for this is the very low stability of purely aqueous sols. Most sols are therefore prepared using at least certain amounts of alcohol or acetone. If such sols are to be applied, the finishing machine has to be suitable for using flammable liquids. Even though the application of flammable finishing agents is not popular in the textile industry it can be carried out easily (if suitable machinery is available). In cooperation with companies from the textile industry, the author has implemented such applications several times on an industrial scale using purely alcohol-based sols. In a successful industrial process a large amount of the solvent can be recycled, the exhaust air can be used for

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energy/heat generation and the quantity of sewage produced is small. Owing to the lower heat capacity of organic solvents compared with water the energy consumption of the drying/curing processes will also be reduced. Nevertheless, the availability of suitable machinery is certainly a drawback and much research has been carried out to increase the stability of sol-gel systems with lower solvent content – this is not specific to the demands of textile industries since many other industries have the same requirements. An interesting approach is used by a company that offers a sol-gel-based product as a binder for textile applications. They supply a sol with a comparatively high concentration of ethylene glycol that is stable for a month. Before application this sol can be diluted with water until the organic solvent proportion is below a critical value. The resulting, mainly aqueous, sol will exhibit only a limited stability, but this stability (at least several hours) is adequate to guarantee the industrial application. Most nanosols are transparent liquids showing a low viscosity comparable with that of water. The reason for this is that most sols applied have a comparably low solid content. This low solid content is, on the one hand, necessary to guarantee sufficient stability of the sols (even in alcohol); as described above, the tendency to gel is dependent on, among other factors, the concentration, thus to prevent unexpected gelling the concentration of the sols is low. On the other hand, one has to consider that the flexibility of a coating is directly related to its thickness. Since the sol-gel-derived materials discussed here all have a relatively high content of inorganic material, the flexibility – compared with an organic polymer – is low. In order to maintain the flexibility of a textile substrate coated with a nanosol it is therefore essential to apply thin coatings; this is achieved by, among other methods, the application of sols with a low solid content. The thickness of the coatings applied to the fiber surface is mainly in the range of several nanometers to micrometers. Crack-free coatings thicker than about 500 nm can only be achieved if the matrices are modified with organic material (Brinker and Scherer, 1990). During the curing process the evaporation of the solvent leads to the development of tensions within these coatings that will lead to cracks if the coating thickness is too high. The modification with organic cross-linking groups will lower the rigidity of the matrix, therefore crack formation can be prevented to a certain extent. Figure 9.6 shows the increase in stiffness and mass per unit area of a finished fabric with increasing solid content. For many applications the stiffness or the flexibility or the handle of a fabric are important. Customization of the sol-gel matrices might therefore be necessary if sufficient values cannot be achieved by lowering the coating thickness (or if a certain thickness is essential for any other reason). In these cases, modification of the sol composition using softeners, polysiloxanes or organic cross-linkers is a reasonable approach to follow to adjust the coating’s properties (Mahltig and Textor, 2008). The increased bending stiffness of the fabrics is not only caused by the rigidity of the sol-gel coating. In the case of coatings prepared from sols with a

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9.6 Relationship between the solid content of a simple silica sol applied to a cotton or polyester fabric and the change in (a) mass per unit area and (b) the stiffness in bending, B, (cotton only). PET, polyethylene terephthalate.

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9.7 SEM micrographs of different polyester fabrics coated with an inorganic nanosol of differing solid content: (a) cracks occur if the coating applied becomes too thick; (b) a bare area forms if a large bridge between two fibers breaks; (c) as panel (b) but without the development of bare areas; (d) sufficiently coated sample.

comparatively high concentration of solids, bridging between adjacent fibers occurs. Figure 9.7 shows scanning electron microscopy (SEM) micrographs of different fabrics treated with either a sol of sufficient solids concentration or a sol with too great a concentration of solids. The number of bridges that can be observed between adjacent fibers increases with increasing concentration. The formation of small bridges can be tolerated to a certain extent – these will break during use, leaving small fins (Fig. 9.7(c)). Larger bridges may cause problems, since during stress an exfoliation of the coating material can be observed, leaving the bare fiber surface (Fig. 9.7(b)). Such defects are critical, for example, for barrier coatings that are applied to protect fiber material in aggressive atmospheres or for water- or oil-repellent finishes. The next section describes in more detail how sufficiently modified sol-gel coatings can be employed to prepare highly repellent textiles. This chapter is intended to give an overview of how sol-gel-derived coatings without particular modification might influence textile properties. We therefore investigated changes in the wetting behavior and the absorptivity of fabrics treated with simple silica

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nanosols. One sees that even highly diluted sols with solid contents in the range of only 1 wt% strongly influence the absorptivity of fabrics. Figure 9.8(a) shows the water uptake of different polyester fabrics that were padded with a simple silica sol, the concentration of the sol was varied between 1 and 20 wt%. It can be seen that the water uptake is greatly reduced after applying the highly diluted silica sols, and an increased concentration will not necessarily lead to a greater decrease in the uptake. Figure 9.8(b) illustrates the results of a capillary rise test (the lower edge of a hanging fabric is immersed into water for a specified time) for a fabric treated with the same sols. While for the grey fabric the liquid rises about 15 cm within the given time period, the liquid rise for the fabric treated with a sol of 2 wt% is only about half this amount and the fabric treated with a 20 wt% sol does not soak up any liquid at all. The fact that a simple silica coating will lead to a higher surface energy compared with an untreated polyester fiber means that the distinctly decreased absorptivity must mainly be explained by changes in the capillary forces. Commercially available aqueous silicic acids (e.g. Levasil®, H.C. Starck; Köstrosol®, Chemiewerk Bad Köstritz or Snowtex®, Nippon Chemical) have been employed as antislip treatments for decades. This means that sol-gel techniques have also been used in the textile industry for decades. The simple inorganic silica sols discussed here also lead to an increased pull-out force of single fibers, which can be taken as a measure of the antislip properties (Berndt, 1983). Furthermore, the formation of bridges, as described above, will lead to a great increase in the fiber pull-out force, which is trivial. Investigations of the effect on tensile strength of the application of simple nanosols to fabrics based on different polymers showed a slight increase in the corresponding values that had previously been demonstrated for sols with low solid content; a significant increase with increasing solid content has not been observed (Mahltig and Textor, 2008). The elongation at break decreased constantly with increasing sol concentration. The changes in the elongation values for sols with a solid content of 20 wt% were in the range of less than 10% in the underlying studies. The same studies showed that coatings derived from less concentrated sols improve the crease recovery angles, e.g. of a polyester fabric, while higher concentration will yield values comparable with the grey fabric. Most sols will only have a negligible effect on the appearance of a modified fabric. Measurements of the degree of whiteness, using the Berger criterion, showed insignificant changes irrespective of the sol concentration. Certain sols will nevertheless change the appearance to a certain extent. For coatings based on titania sols a slight yellowing is observed, while sols based on colored metal oxides will, as is to be expected, change the color distinctly. In conclusion, the author would like to describe an initial surprising observation with respect to the topic flame retardation. Many applications demand flameretardant material, different authors have discussed the preparation of sol-gel-based coatings that improve this retardance. These approaches are based on the

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9.8 (a) Water uptake of three different polyester fabrics after treatment with simple silica sols of different solid content (lines are only a guide for the eye). (b) Capillary rise method, comparision of untreated cotton fabric (right column) and cotton fabric treated with 20 wt% (left column) and 2 wt% (middle column) silica sol.

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9.9 A Trevira CS® fabric after being set alight with a gas torch. The lefthand sample is the grey sample, the right-hand sample was coated with a silica sol.

modification of nanosols with a variety of industry established additives such as phosphate esters (Song et al., 2005), orthophosphoric acid or diammonium hydrogen phosphate (Chapple and Ferg, 2006; Horrocks et al., 2000). In the light of this demand for flame-retardant materials an unexpected observation can be made: the completely inorganic nanosols will be fireproof, therefore one might expect that the modification of flame-retardant textiles with an inorganic nanosol will not change the corresponding properties of the textile. In fact it might – Fig. 9.9 shows a Trevira CS® fabric (flame retardant!) that was set alight, the left-hand sample is the grey fabric, the right-hand sample was modified with a silica sol. The grey fabric cannot be ignited, it simply chars, the coated fabric continues burning after removing the flame. A possible explanation might be that the thin inorganic coating stiffens the fabric/fibers and prevents it from rolling up, as usually happens when the polymer is in contact with a flame; this maintains a high surface area which is essential for flame propagation. The sol-gel approach is a versatile technique that can be employed for modification of many textile materials – in the literature one can find reports of almost every type of fiber material modified with a huge number of different sols. It really seems to be possible to finish almost any textile material following the general approach. The authors tested natural fibers such as cotton and wool, and those synthetics commonly used in the textile industry (for example, polyester, polyamides, aramides and polyacrylonitrile); fabrics made of polyolefines or even polytetrafluoroethylene have been successfully equipped with sol-gel derived finishes. As this experience shows, it can be stated that natural fibers in particular, but also polyamides- or polyacrylonitrile-based materials, can be modified with

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comparably simple sols yielding excellent durability. Depending on the specific application and the fiber polymer, the sols will need to be adapted to improve or provide satisfactory wetting, homogeneity and especially durability. Such adaptations can, among others, be achieved by introducing additives that establish a covalent linkage of the sol-gel derived matrices to the polymer surface or by employing primers. At the same time, a physical pretreatment of the textiles might be advantageous to increase the polarity of the polymer surface and/or to establish functional groups that improve the interaction between the matrix and the polymer. Such treatments could include, for example, plasma or corona treatment or photochemical activation. Finding a suitable technique for evaluating the adhesion between the solderived layers and the textile substrate is not trivial. With respect to the textile structure, and since the applied coatings are extremely thin, commonly used peeling tests appear to be unsuitable. Tests with adhesive tapes might give certain information, but since the coatings are mostly colorless, a simple visual evaluation will fail; it is therefore necessary to employ at least optical or scanning electron microscopy. For conventional polymer coatings adhesion forces are also investigated by embedding single fibers or yarns into the polymers and measuring the pull-out force. Owing to the fact that the primary inorganic sol-gel-derived materials show an extreme shrinkage during curing – followed by the aforementioned cracking of the thicker samples – it is mostly impossible to prepare corresponding samples. Probably the most promising results can be achieved by investigating the adhesion indirectly – successful wear-resistance tests (e.g. the Martindale test), bending tests or, for example, laundry tests will certainly not measure the adhesion forces but will at least prove sufficient adhesion.

9.3

Sol-gel-based finishing effects

The section above introduced the reader to the basic aspects of the sol-gel technique and some general aspects of treating textile materials were discussed. This section will now describe selected modifications or functionalizations that can be achieved by an appropriate modification of initially relatively simple sols. The examples described are only a sample of what is possible and what has been investigated; a very detailed review, Nanosols & Textiles, has recently been published by Mahltig and Textor (2008).

9.3.1

Water repellence, oil repellence, self-cleaning properties

In the above section we described how simple nanosols can influence the wetting behavior of a treated textile. Topics such as water and oil repellence, and selfcleaning properties, are probably as old as the use and production of textiles,

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Methyltriethoxysilane

Hexadecyltriethoxysilane

Octyltriethoxysilane

Tridecafluorooctyltriethoxysilane

9.10 Selection of additives that have been used to prepare hydrophobic or even oleophobic sol-gel coatings.

nevertheless the investigation of corresponding finishing materials is still of enormous interest. The sol-gel approach offers far-reaching possibilities for the development of promising solutions. As seen in Section 9.2, a simple nanosol can already reduce or decelerate the water uptake of a treated fabric, mainly by changing the capillarity of the substrate. Improved repellence can not be expected because the surface energy of these coatings is similar to that of the fiber polymer or even higher. An appropriate modification of the basic formulation allows one to prepare sol-gel-derived matrices with reduced surface energies, thus improving the repellence properties. A variety of compounds can be added to a simple nanosol to yield highly repellent coatings. Appropriate additives might be functional siloxanes, hydrophobic alkoxysilanes or, among others, certain fluorinated monomers. A selection of useful components is shown in Fig. 9.10. Sufficient oil repellence can only be achieved by employing fluorinated compounds. Occasionally oil-repellent, fluorine-free materials are advertised as being at least as good as conventional fluorocarbon finishes, but as far as the author knows, up to now none of these materials have behaved as promised. One advantageous aspect of the sol-gel approach is that the amount of hydrophobic additives necessary to achieve an adequate repellence is very low. Figure 9.11 shows the results of a corresponding investigation (Mahltig and Textor, 2008). A simple hybrid polymer sol was modified with different amounts of a fluorinated alkoxysilane. Measurements of the contact angle of water on the resulting coatings showed that a proportion of at most 2 vol% of the hydrophobic component is necessary to achieve maximum contact angles. The explanation found in the literature (Fabbri et al., 2006) for the low amounts of hydrophobic additive required to achieve a strong effect is that owing to a gradient in the interfacial tension of the different components of the sol, the components with the lower interfacial tension will concentrate near the surface during the gelation process. This effect reduces the amount of expensive

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9.11 Contact angle of a coating based on a simple hybrid polymer sol modified with different amounts of a highly fluorinated alkoxysilane.

fluorochemicals to a minimum, and simultaneously the problem of adhesion to the substrate might be reduced due to the interface tension gradient within the applied coating layer. The fluoropolymers, in particular, that are conventionally used to prepare highly repellent surfaces are known to be very soft, the combination of the sol-gel matrix with the low amounts of hydrophobic matrix elements promises the availability of repellent surfaces with improved durability. Figure 9.12 shows the repellence of a fabric treated with a standard sol that was modified with different additives, yielding repellency against various liquid contaminants. While aliphatic additives will basically repel only the aqueous contaminants a fluorinated additive will also protect against hydrophobic substances. A low surface energy is one of the keys to a highly repellent surface. Superrepellent surfaces can only be achieved in combination with a particular topography. This has been generally known and understood for the last 60 years, mainly owing to the work of Cassie and Baxter (Cassie and Baxter, 1944; Cassie, 1948) as well as that of Wenzel (1936). Cassie and Baxter showed that a combination of a particular micro-roughness and low surface energy will increase repellent properties in comparison with the same but flat substrate. (The above-mentioned

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Surface modification of textiles Standard sol modified with: Control

alkylsilane

fluorosilane

fluorsurfactant

Colored water Coffee Red wine Colored oil

9.12 Wetting behavior of different test liquids applied to cotton fabrics treated with nanosols modified with different hydrophobic additives. Boris Mahltig, GMBU, Dresden, Germany is thanked for providing the photographs of the different samples.

work of Cassie and Baxter is fundamental, and therefore it should be stated here that A. B. D. Cassie was a textile researcher!) The explanation for the LotusEffect, first described by Barthlott and Neinhuis (1997), is also based on the above-mentioned work. One approach to the preparation of super-repellent coatings is the modification of hydrophobic nanosols with either strongly agglomerated nanoparticles (e.g. Aerosil® particles) or large particles (in the range >500 nm) yielding sufficiently structured and highly hydrophobic surfaces. Figure 9.13 shows a wool sample that was coated with a highly hydrophobic sol modified with Aluminium C (alumina particles prepared by the Aerosil process). While the grey fabric is only hydrophobic (owing to the hydrophobic wool waxes) the sol-gel-treated sample is super-repellent.

9.3.2

Protection against aggressive media

Materials such as polyester, glass fiber and other fibers are known to be degraded under basic conditions, while polyamides, for example, degrade under acidic conditions. One promising approach for textiles that are employed in aggressive

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9.13 Repellence of a wool sample either as-received (right) or treated with a hydrophobic sol-gel coating modified with a nanopowder (left).

atmospheres is the application of protecting barrier coatings. Such barriers can not only protect against degradation but also against swelling of the fiber material in the presence of water or organic solvents, or can prevent the migration of certain substances either into or out of the fiber. Such substances might be, among others, contaminants, softeners or fragrances. In addition to the prevention of a contact between the fiber polymer and certain substances, appropriate barriers might be needed to prepare protection textiles intended primarily for protective equipment for civilian and military forces. If properly prepared and applied, sol-gel coatings applied to textile materials mostly build up thin dense layers covering every single fiber. Thus the use of solgel coatings for the preparation of barrier coatings seems to be appropriate. Figure 9.14 shows an example of a glass fiber material that was coated with a hydrophobic hybrid polymer sol. Glass fiber is known to show comparatively low resistance in basic atmospheres, since the glass fiber is hydrolyzed under these conditions, leading at least to weakening of the fiber material. The fiber material investigated for the results summarized in Figure 9.14 was fixed hanging in a closed vessel that contained a beaker filled with concentrated ammonia solution. The samples were stored for up to 2 weeks at a temperature of 60 ºC in the resulting atmosphere. As can be observed, the grey fabric loses its tensile strength within a

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9.14 Tensile strength of an as-received glass fiber fabric (circles) and a fabric coated with a sol-gel coating (squares) after storage in a saturated ammonia/water vapor atmosphere (Textor et al., 2003).

short time, after 2 weeks the strength was reduced by more than 80%. The sample coated with a sufficient sol-gel coating did not suffer any significant weakening. These results prove two points. They not only show that the coatings prepared can act as an effective barrier, but also show that the thin coatings, applied by a simple padding process, cover the fiber material almost perfectly. If this was not the case, bare fiber areas would have been in contact with the aggressive atmosphere – gaseous ammonia and water vapor are expected to reach even those areas that are not easily accessible; this would result in, at the least, local fiber decomposition/hydrolyzation and therefore inevitably lead to a reduced tensile strength, which was not observed. These results are shown to point out that the application of appropriate sol-gel-derived coatings, applied with techniques commonly used in textile industries, is a promising approach not only to protect fiber materials against a variety of aggressive atmospheres but also for a variety of other benefits as will be discussed in the following section.

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Light absorption – ultraviolet protection, color, photochromic effect

As mentioned above, simple nanosols will at most influence the appearance of a finished textile only slightly and a silica-based coating will show no significant absorption either in the UV region or in visible light. In spite of this, many modifications have been reported that yield different kinds of changes in the light absorption. Since the appearance of the hole in the ozone layer the UV intensity of solar radiation has increased. Clothing has always been used to protect people against environmental influences and protection against skin cancer caused by UV radiation is of growing importance. For fashion reasons people prefer lightweight and light-colored clothes – unfortunately these offer the lowest protection against the hazardous radiation. The absorption spectra of semiconductors such as titania or zinc oxide show strong absorption in the UV region of the light spectrum but only very slight or no absorption of visible light. These materials show a high extinction coefficient for UV light and a steep drop in the absorption curve at the transition from UV to visible light. In comparison with the organic absorbers conventionally used in the textile industry, inorganic materials show no significant degradation and are therefore extremely stable and the oxides are classified as nontoxic materials. Titania as well as zinc oxide are harmless, that is why both are used in cosmetics such as suncream. For the above-mentioned reasons, titanium or zinc oxide seem to be ideal for the preparation of highly UV-absorbing, sol-gel-based coatings. Although skin protection has been the focus of research to date, it should be borne in mind that an efficiently absorbing coating might also be suitable for the protection of UV-sensitive fiber polymers or for increasing the light-fastness of textiles. Titania and zinc oxide nanoparticles are commercially available (from Evonik, Germany and Sachtleben, Germany, among others) or can be easily prepared by sol-gel-based techniques. The particular oxides can be mixed into a standard solgel formulation which is then applied to a textile by a simple padding process. Figure 9.15 shows the UV–vis spectra of a coating based on an epoxysilane that was modified with about 10 wt% zinc oxide. As can be seen, the basically transparent coating will show strong absorption up to approximately 400 nm and only slight absorption at longer wavelengths. The slight absorption of visible light measured is rather the result of a scattering of the zinc oxide particles. If the particles are not dispersed ideally the coating will show many agglomerates of several hundred nanometers or even micrometers in size. These coarse particles scatter light, causing the apparent slight absorption. This effect can be avoided by preparing better dispersions or, for example, by combining stable titania or zinc oxide sols with a suitable size of oxide particles. Theory shows that particles with a size smaller than approximately 50 nm will not scatter visible light. Figure 9.16 shows the results of measuring the ultraviolet proection factor

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9.15 UV spectra of a polyethylene film coated with an unmodified epoxysilane sol (dotted line) and a film coated with the same sol but modified with zinc oxide (dashed line).

(UPF) values of differently treated textiles; as can be seen, dyeing a textile with a dark color or employing organic UV absorbers increases the protection factor significantly but best results here are achieved by a colorless coating filled with zinc oxide. Comparable results can be found for titania but it should be mentioned that these coatings sometimes yield a slight yellowing. The sol-gel technique also offers an easy approach for dyeing textile materials. Many investigations have been carried out modifying simple nanosols with conventional dyestuffs employed in the textile industry. The particular advantage of this approach is that simple dyeing systems would be available that can be ideally used to dye not just a single fiber polymer but many different ones. The results reported up to now, for example by Mahltig et al. (2004a, 2004b),and Trepte and Böttcher (2000), show promising results. The dyestuffs are mixed with or dissolved in a suitably adjusted sol and the textiles are prepared by a simple padding process. The adjustment of the sols is necessary to improve the immobilization of the dyestuffs within the sol-gel-derived coatings. These modifications can be carried out to guarantee sufficient ionic interaction between the dyestuffs and the binder, or to establish covalent binding. Up until now the

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9.16 UPF values of differently treated textiles compared with those achieved by a sol-gel coating modified with zinc oxide nanoparticles. The fabrics were either simply dyed in yellow or blue or the treatment combined the dyeing and the application of an organic UV absorber (measurements were carried out by Yvette Dietzel at the STFI, Chemnitz, Germany).

reported results have not been able to compete with conventional dyeing with respect to the washing fastness, thus the sol-gel dyeing technique will certainly not replace conventional dyeing. Nevertheless the described approach might be advantageous for certain applications since it is simple and versatile. Furthermore, several authors report (e.g. Mennig et al., 1999) that the light-fastness of dyestuffs increases due to the incorporation into the sol-gel matrices, explained by protection of the organic molecules within the inorganic pores/cavities. An exotic example prepared by the general approach is given in Fig. 9.17, which shows a polyester fabric that was coated with a sol-gel coating modified with a photochromic dyestuff. This coating is initially colorless but changes its color to blue if irradiated with UV light. In order to visualize this change the fabric was masked with the word ‘NANO’ during irradiation. The picture was taken immediately after the irradiation, after about a minute the color disappeared. The fabric was placed in

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9.17 Polyester fabric, coated with a photochromic coating, that was then irradiated with UV radiation. During irradiation the sample was partially covered with the word ‘NANO’. The picture was taken immediately after irradiation, the darker (blue), irradiated areas will fade within minutes.

boiling water for about 1 hour and this did not lead to any visible changes when the fabric was irradiated again, proving at least a certain durability.

9.3.4

Improved wear resistance

Possibly the first commercial employment of inorganic–organic hybrid polymers was as a scratch-resistant coating for plastic lenses or compact disks (Haas et al., 1999). These hybrid polymers are basically prepared from organically modified alkoxysilanes and are modified with metal oxide nanoparticles with particle sizes well below 50 nm to guarantee sufficient stability against mechanical impact. The literature reports that the improved mechanical properties are associated with the formation of a composite material on a quasi molecular level. The interface between the matrix-forming alkoxysilane and the nanoparticles is extremely large if the particles are sufficiently dispersed. The use of microparticles will, therefore, not yield comparable improvements. On the other hand, as mentioned above, large particles would scatter light, leading to at least opaque coatings, which would not only impair the appearance of a textile, but would also be intolerable for a plastic lens such as a spectacle lens. It is clear that simply transferring an effect that is suitable for a spectacle lens to a textile is not possible. The requirements of textile materials, mainly with regard to flexibility, are completely different to those of the lenses and experience shows that the scratch resistance of the coated lenses shows at least a certain correlation with the hardness and flexibility of the coating

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9.18 Glass fiber samples after carrying out a Martindale test. The grey glass fiber fabric (left) was treated with 100 scrubbing cycles, the coated fabric (right) with 10 000 cycles.

material. Nevertheless it has been shown that comparable coatings, modified with respect to an improved flexibility, yield excellent wear resistance. Figure 9.18 shows a glass fabric that has undergone Martindale abrasion tests; while the grey fabric is distinctly damaged (or completely destroyed), the coated samples show less or no damage even if the duration of the abrasion test was prolonged. The flexibility or the hand of the fabrics are certainly influenced by the coatings applied, which is why the samples presented here would normally be used in technical textiles.

9.3.5

Photocatalytic coatings

Nowadays self-cleaning properties are mostly discussed in the context of the Lotus-Effect®. The Lotus-Effect® has a number of drawbacks. On the one hand, if self-cleaning includes oil repellence the finishing has to be based (at least partially) on fluoro-chemicals and the surface has to exhibit a specific micro- and nanostructure. On the other hand, even if an adequate finishing has been achieved, the structured surfaces are comparably soft which might be a problem in terms of abrasion stability. Materials like anatase – which is a particular crystal structure of titania – show a so-called ‘photocatalytic’ behavior. During irradiation with UV light the semiconductor will show a certain activity. The excited anatase is able to oxidize hydroxyl ions to hydroxyl radicals. These radicals are an extremely reactive species that are able to initiate the decomposition of most organic materials. Furthermore, the excited anatase is able to oxidize or reduce numerous organic materials directly leading also to the decomposition of these materials and

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9.19 Polyester fabrics modified with a photocatalytic coating were placed in Petri dishes containing dyestuff solution (methylene blue). The right-hand dish was irradiated with UV light yielding a complete discoloration of the solution due to a photocatalytic decomposition of the dyestuff molecules.

to the formation of radicals, forcing further decomposition reactions. A more detailed description of the reactions taking place if anatase is irradiated in the presence of humidity and organic materials is given in Bozzi et al. (2005) and Nosaka et al. (1998). Analogous to the situation described above, it is also possible to modify simple nanosols as well as inorganic–organic hybrid polymers with anatase nanoparticles. Anatase nanoparticles are available commercially (for example Aeroxide P25®, Evonik, Germany) and can be dispersed into a starting sol and applied to a textile by padding. The advantage of the commercially available anatase nanopowders is basically the high proportion of crystalline anatase and the resulting high activity of the particles. However, one drawback is certainly the more laborious process necessary for dispersing the particles in the sols. An alternative is the preparation of titania nanosols, the correctly prepared sols do not need an extra dispersion process and can be simply mixed with another sol that acts as a binder or the titania sol can be directly applied, forming a pure titania coating. The drawback – to date – is the fact that the content of crystalline anatase in the sol-gel-derived particles is comparably low, but several groups are working to achieve higher crystallinity. Figure 9.19 shows two Petri dishes that were filled with a dyestuff solution before polyester fabrics were placed into the dish. During irradiation with UV light the dyestuff will be decomposed when the fabric sample has been modified with a coating exhibiting the photocatalytic effect. The same can be observed if the samples are exposed to sunlight when stored on a windowsill (inside!).

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The dyestuff solutions are certainly only a model system for possible contaminations that are deposited on textile materials in use. Such coatings still have to prove whether they show a sufficient speed or capacity for degradation that guarantees a constantly clean product under all conditions of usage. One should, in addition, consider that degradation of the fiber polymer coated with the active coatings is possible – the same radicals that decompose the contaminants could also decompose the textile materials. Some investigations have been carried out with respect to this question and have shown at least a very slight and negligible acceleration of the fiber decomposition. No accelerated bleaching of colored/dyed textiles coated with such systems was observed in the investigations carried out to date by Textor et al. (2007, 2008).

9.4

Future trends

The principles of the sol-gel technique and some general aspects of the coating or finishing of textiles have been introduced. Furthermore, several concrete approaches have been discussed in more detail showing the enormous potential of the sol-gel technique if it was transferred into the textile industry. Many further features that can be realized by suitable sols are discussed in the literature but cannot be described completely within a chapter such as this, since the number of groups working on this topic is enormous. Many of these reports only mention the modification of textiles to explain additional fields of application and do not describe any ‘textile experience’. Nevertheless, more than a few have realized the enormous potential in relation to textile industries and therefore have concentrated their work on textiles. Very interesting work has been published with respect to antimicrobial finishings for textile materials (Betancor et al., 2005; Blaker et al., 2004; Böttcher et al., 1997, 2994; Mahltig et al., 2004c), this is of enormous interest not only for the growing market of medical textiles but, for example, for outdoor applications that suffer from biofouling in use. The antimicrobial coatings are only a part of the wide field of prospective applications that sol-gel-derived coatings offer for biotechnological products – the coatings can either act as a simple binder for bio-active materials or the specific (xero-)gels prepared may exhibit intrinsic biological activities. The (surface) conductivity of textiles is of particular importance for several reasons. One is the prevention of electrostatic charging, another is the growing demand for textiles combined with electronics. Sol-gel-derived solutions for equipping textiles with antistatic properties (surface conductivities below 109 ohm) are of interest for home textiles, but also particularly for working clothes and technical textiles employed in industrial environments, where electrostatic charging represents an enormous hazard. Conventional finishings show low durability and lead to an increased soiling. The fact that a single sol-gel-derived coating can be easily equipped with different properties leads to the hope that effective and durable antistatic finishings are achievable that simultaneously show

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promising repellence (Mahltig and Textor, 2008). The development of, for example, textile sensors or other combinations of textiles with microelectronic properties leads to an increasing demand for coatings/fibers with a significant conductivity, much lower than the above-mentioned 109 ohm; modification of simple nanosols could meet these expectations. Future sol-gel research will unquestionably discover many new applications of sol-gel-derived coatings for finishing textile materials. Numerous properties will most probably become available in combination with textiles that we cannot foresee today. From the author’s point of view there are two crucial aspects that have to be focused on to really establish the general technology in textile industries successfully. One aspect is that future efforts will have to focus on a transfer from sols based on organic solvents (mainly alcohols, acetone) to water-based systems because the present-day textile industry is geared to processing aqueous products. Sol-gel chemistry is usually carried out in solvents because the stability of most sols (especially in higher concentration) in water is low. The development of approaches to provide stable, water-based systems (or at least systems with low amounts of solvent) is therefore an important task. The first sol-gel-based systems developed for textile application are now commercially available. These systems consist of a concentrated alcohol-based sol that is highly diluted with water before use. The alcohol-based sol achieves a sufficient storage time; after dilution the sol will exhibit only a low storage stability but enough to carry out the application process. Nevertheless, and this is the second aspect mentioned above, it can be expected that the development of solvent-free (solvent-reduced) sols will not be successful for all applications and for certain applications it might, therefore, be necessary to motivate the textile industry to employ, for example, alcohol-based sols. Corresponding production tests have been carried out successfully using a stenter frame adapted to solve the problem of explosion prevention. Besides the undeniable disadvantages of this approach one should also consider the advantages, i.e. the energy consumption in the drying process will be reduced as will water consumption, the solvent can ideally be recycled and exhaust air could be used for heat production. The textile industry, especially in Western Europe, has managed to move successfully from a traditional industry predominantly producing and finishing clothes and home textiles to a high-tech industry producing products for the automotive or aerospace industries as well as for the medical sector. In the author’s view, this industry will also be able to employ solvent-based finishings if the advantages of sol-gel-based products can outweigh the increased operating expenses.

9.5

References

BARTHLOTT W AND NEINHUIS C (1997), Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta, 202, 1–8.

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BERNDT H-J (1983), Die Messung der Ausziehkraft eines Fadens, eine einfache Methode zur Bestimmung des Schiebeverhaltens von Geweben, Melliand Textilber., 12, 917 (in German). BETANCOR L, LÓPEZ-GALLEGO F, HIDALGO A, FUENTES M, PODRASKY O, KUNCOVA G, GUISÁN J M AND FERNÁNDEZ-LAFUENTE R (2005), Advantages of the pre-immobilization of enzymes on porous supports for their entrapment in sol-gels, Biomacromolecules, 6, 1027–1030. BLAKER J J, NAZHAT S N AND BOCCACINI A R (2004), Development and characterisation of silver doped bioactive glass-coated sutures for tissue engineering and wound healing applications, Biomaterials, 25, 1319–1329. BÖTTCHER H, KALLIES K-H AND HAUFE H (1997), Model investigations of controlled release of bioactive compounds from thin metal oxide layers, J. Sol-Gel Sci. Technol., 8, 661–654. BÖTTCHER H, SOLTMANN U, MERTIG M AND POMPE W (2004), Biocers: ceramics with incorporated microorganisms for biocatalytic, biosorptive and functional materials development, J. Mater. Chem., 14, 2176–2188. BOZZI A, YURANOVA T, GUASAQUILLO I, LAUB D AND KIWI J (2005), Self-cleaning of modified cotton textiles by TiO2 at low temperatures under daylight irradiation, J. Photochem. Photobiol. A: Chem., 174, 156–164. BRINKER C J AND SCHERER G (1990), Sol-Gel Science: The Physics and Chemistry of SolGel Processing, Academic Press, Boston, MA. CASSIE A B D (1948), Contact angle, Discuss. Faraday Soc., 11, 11–16. CASSIE A B D AND BAXTER S (1944), Wettability of porous surfaces, Faraday Soc., 40, 546– 551. CHAPPLE S A AND FERG E (2006), The influence of precursor ratios on the properties of cotton coated with a sol-gel flame retardant, AATCC Rev., 6, 36–40. DAOUD W A AND XIN J H (2004a), Nucleation and growth of anatase crystallites on cotton fabrics at low temperatures, J. Am. Ceram. Soc., 87, 953–955. DAOUD W A AND XIN J H (2004b), Low temperature sol-gel processed photocatalytic titania coating, J. Sol-Gel Sci. Technol., 29, 25–29. FABBRI P, MESSORI M, MONTECCHI M, NANNORONE S, PASQUALI L, PILATI F, TONELLI C AND TOSELLI M (2006), Perfluoropolyether-based organic-inorganic hybrid coatings, Polymer, 47, 1055–1062. HAAS K-H, AMBERG-SCHWAB S AND ROSE K (1999), Abrasionsbeständige Antistatik-und Antihaftschichten auf der Basis von anorganisch-organischen Hybridpolymeren (ORMOCERen), Jahrbuch Oberflächentechnik, 55, 183–198 (in German). HORROCKS A R, WANG M Y, HALL M E, SUNMONU F AND PEARSON J S (2000), Flame retardant textile back-coatings. Part 2. Effectiveness of phosphorus-containing flame retardants in textile back-coating formulations, Polym. Int., 49, 1079–1091. HUANG C, WANG H, FANG L-N, WANG H AND ZHANG H-J (2006), Finishing of cotton fabrics with nanometer ZnO and chitosan, J. Textile Res., 27, 41–44 (in Chinese). IWASHITA C, MITANI Y AND ASAMI C (2000), Process for the preparation of thread, string, rope or woven fabric with photocatalyst for decomposing organic compounds, European Patent, EP1008565. LI Z-R, XU H-Y, FU K-J AND WANG L-J (2007), ZnO nanosol for enhancing the UVprotective property of cotton fabric and pigment dyeing in a single bath, AATCC Rev., 7, 38–41. LIUXUE Z, XIULIAN W, PENG L AND ZHIXING S (2008), Low temperature deposition of TiO2

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thin films on polyvinyl alcohol fibers with photocatalytical and antibacterial activities, Appl. Surf. Sci., 254, 1771–1774. MAHLTIG B AND TEXTOR T (2008), Nanosols and Textiles, World Scientific Publishing Co., Singapore. MAHLTIG B, KNITTEL D, SCHOLLMEYER E AND BÖTTCHER H (2004a), Incorporation of triarylmethane dyes into sol-gel matrices deposited on textiles, J. Sol-Gel Sci. Technol., 31, 293–297. MAHLTIG B, KNITTEL D, SCHOLLMEYER E AND BÖTTCHER H (2004b), Light fading and wash fastness of dyed nanosol-coated textiles, Textile Res. J., 74, 521–527. MAHLTIG B, FIEDLER D AND BÖTTCHER H (2004c), Antimicrobial sol-gel coatings, J. SolGel Sci. Technol., 32, 219–222. MENNIG M, FRIES K AND SCHMIDT H (1999), Photochromic organic-inorganic hybrid nanocomposite hard coatings with tailored fast switching properties, in Proceedings of the Spring Materials Research Society Symposium, 576, 409–414. NOSAKA Y, KISHIMOTO M AND NISHINO J (1998), Factors governing the initial process of TiO2 photocatalysis studied by means of in-situ electron spin resonance measurements, J. Phys. Chem. B, 102, 10279–10283. QU M, LIU A AND SUN H (2002), Barium sulfate sol for modifying polyester and its preparing process, Chinese Patent, CN1365993. SONG L, HU Y, TANG Y, ZHANG R, CHEN Z AND FAN W (2005), Study on the properties of flame retardant polyurethane/organoclay nanocomposite, Polym. Degrad. Stab., 87, 111– 116. TEXTOR T, KNITTEL D, BAHNERS T AND SCHOLLMEYER E (2003), Inorganic-organic hybrid polymers for coating textile materials, Curr. Trends Polym. Sci., 8, 127–133. TEXTOR T, SCHRÖTER F AND SCHOLLMEYER E (2007), Thin coatings with photo-catalytic activity based on inorganic-organic hybrid polymers modified with anatase nanoparticles, Macromol. Symp., 254, 196–202. TEXTOR T, SCHROETER F AND SCHOLLMEYER E (2008), Photocatalytic titania derived by sol-gel technique for textile application, in Silanes and Other Coupling Agents, Vol. 5, Ed. K. L. Mittal, VSP, Leiden, pp. 305–322. THOMAS S, SAKTHIKUMAR D, JOY P A, YOSHIDA Y AND ANANTHARAMAN M R (2006), Optically transparent magnetic nanocomposites based on encapsulated Fe3O4 nanoparticles in a sol-gel silica network, Nanotechnology, 17, 5565–5572. TREPTE J AND BÖTTCHER H (2000), Improvement in the leaching behavior of dye-doped modified silica layers coated onto paper or textiles, J. Sol-Gel Sci. Technol., 19, 691– 694. WENZEL R N (1936), Resistance of solid surfaces to wetting by water, Ind. Eng. Chem., 28, 988–994. WU C-C, YANG H, FENG B AND LU W-W (2004), Research on the characteristics of antimony doped tin oxide (ATO)-SiO2 composite thin film by sol-gel, Electron. Compon. Mater., 23, 22–24 (in Chinese). XIN J H AND DAOUD W A (2005), Method of providing a coating of titanium dioxide to an article and the article with this coating, World Patent, WO2005113443. XIN J H, DAOUD W A AND KONG Y Y A (2004), New approach to UV-blocking treatment for cotton fabrics, Textile Res. J., 74, 97–100. XU P, WANG W AND CHEN S-L (2005a), UV blocking treatment of cotton fabrics by titanium hydrosol, AATCC Rev., 5, 28–31. XU Z Z, WANG C C, YANG W L AND FU S K (2005b), Synthesis of superparamagnetic Fe3O4/

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SiO2 composite particles via sol-gel process based on inverse miniemulsion, J. Mater. Sci., 40, 4667–4669. ZORJANOVIC J, ZIMEHL R, SCHOLLMEYER E, PETRACIC O, KLEEMANN W, KNITTEL D, TEXTOR T AND SCHLOSSER U (2004), Electrostatic and electromagnetic fields – new materials and technologies, Proceeding of the 6th Symposium EL-TEX 2004, pp. 168– 175.

10 Nano-modification of textile surfaces using layer-by-layer deposition methods P . L U and B . D I N G

University of California, USA and Donghua University, China

Abstract: The use of nanotechnology in the modification of textile surfaces has attracted much interest in recent years. In this chapter, we briefly review several common nanotechnologies, including: liquid phase deposition, Langmuir–Blodgett films, the sol-gel technique, physical vapor deposition, chemical vapor deposition, and plasma surface modification. We then focus our interest on the newly developed layer-by-layer (LbL) technique. LbL methods, key principles, mechanisms and experimental considerations for textile materials are introduced and discussed. The LbL technique has been applied to modify the surfaces of both traditional textile fibers and electrospun nanofibers. Furthermore, nanotubes or hollow fibers have been fabricated using the LbL method. These preliminary results have demonstrated the versatility of the LbL technique for the modification of textile surfaces. Key words: nanotechnology, textile surfaces, layer-by-layer techniques, traditional textile fibers, electrospun nanofibers, nanotubes, hollow fibers.

10.1

Introduction

10.1.1 What is nanotechnology? Nanotechnology has been defined as a field of applied science and technology concerned with the thorough three-dimensional (3D) structural control of materials, processes, and devices on an atomic and molecular scale, generally 100 nm or smaller. One of the first to articulate a future rife with nanotechnology was Richard Feynman, a Nobel laureate. In late 1959 at the California Institute of Technology, he presented the nanotechnology community’s founding liturgy which was entitled ‘There is plenty of room at the bottom’. The ability to manipulate individual atoms or molecules and place them in a desired structure leads to a new industrial revolution and completely changes the way most things are constructed. Nanotechnology, though still at an early stage in its development, has already proven to be a very useful tool for improving the performance of textiles and enabling them to be multifunctional. With the increased performance and functions comes added value, and hence additional revenue from textile products. 214

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10.1.2 Nanotechniques for textile surface modification Surface modifications have long been utilized to change the physical and chemical structures of the surface layers of textile fibers to improve the properties of fibers in many applications. The use of nanotechnology in modifications of textile surfaces has attracted much interest in recent years. Various strategies for surface modifications have been widely employed, for example: liquid phase deposition (LPD) (Huang, 2005); Langmuir–Blodgett (LB) films (Chen et al., 2007); the solgel technique (Balasubramanian et al., 2003); physical vapor deposition (PVD) (Singh and Wolfe, 2005); chemical vapor deposition (CVD) (Fahlman, 2006); and plasma techniques (Poncin-Epaillard and Legeay, 2003). The LPD process consists of a ligand exchange equilibrium reaction of the metafluorocomplex ions and F– ions in which the boric acid (H3BO3) or metal aluminum reacts readily with F– ions to form the more stable BF4– ions. With the promoted consumption of non-coordinated F– ions by boric acid as an F– scavenger, it is possible to form a metal oxide thin film on the substrate which is immersed in the treatment solution for deposition without any special equipment (Zhang et al., 2007a). The LPD makes it easy to form metal oxide thin film on various kinds of substrates with large surface area and/or complex morphology at low temperature, usually between room temperature and 100 ºC (Yu et al., 2006). However, the reaction time for the LPD process is very long and the metal (usually titania) is not well crystallized in the preparation of nanocrystal TiO2 films by the LPD procedure (Masuda et al., 2003). A post-treatment of sintering at above 200 ºC is required in order to obtain high crystallinity (Shimizu et al., 1999, Yu et al., 2005). The LB technique is a well-established and sophisticated method for controlling the interfacial molecular orientation and packing. In an LB layer, the orientation of molecules is ordered owing to the amphiphilic character of the materials. In addition, LB layers often show nanostructures such as domains, dots, and wires owing to anisotropic intermolecular interactions. It is an efficient approach toward the controllable fabrication of laterally patterned structures which are normally generated by the deposition of ordered two-dimensional (2D) domains formed at the air–water interface onto solid substrates (Tsunashima et al., 2007). Although the LB method is an elegant way of building up multilayer structures, it suffers from the fact that rather expensive instruments are required and it is not applicable for many kinds of non-amphiphilic materials (Ariga et al., 2007). Sol-gel processes allow the fabrication of solid materials starting either from a stable suspension of particles or from polymer molecules in a liquid (sol) to produce an integrated network (gel) that is enough to immobilize the liquid. The gelation is usually due to the irreversible formation of covalent bonds, which can lead to high-quality ceramic or glass bodies by careful drying and firing. Alternatively supercritical solvent extraction can be used to make highly porous, low-density monoliths. Spin- or dip-coated film gel can be fired during drying to yield dense films; similarly, spray drying can lead to spherical particles and

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spinning with evaporation to fibers. Moreover, in the presence of a template, gelation can also be utilized to design nanoporous materials and other useful nanostructures such as tubes, fibers, helices, and ribbons (Hector, 2007). However, sol-gel methods have some disadvantages for industrial applications because repeated coating is required in order to get a thick film (Yu et al., 2006). PVD is a general term used to describe the deposition of thin films onto various surfaces, such as a semiconductor wafer, by the condensation of vaporized forms of materials. PVD is a purely physical process, for example high-temperature vacuum evaporation. Deposition of thin films by PVD techniques has found wide use in various industrial sectors because it allows the coating of metals, alloys, ceramics, or polymer thin films onto a wide range of substrates (Helmersson et al., 2006). However, PVD requires expensive, high-vacuum equipment and a great deal of energy for the deposition of films. In contrast, CVD is a chemical process in which gaseous precursors are reactively transformed into a thin film, coating or other solid-state material on the surface of a substrate (Fahlman, 2006). CVD is a versatile technology that offers good control of film structure and composition, uniformity, and growth rates. It is widely utilized to manufacture thin solid films in semiconductors and solar cells, antireflection and spectrally selective coatings on optical components, and anticorrosion and anti-wear layers on mechanical tools and equipment. In recent years, CVD has also been applied in the preparation of micro-electro-mechanical systems (MEMS) and nanostructures (Kleijn et al., 2007). As with the PVD process, the CVD process also requires special apparatus and high vacuum conditions, and consumes large amounts of energy. A plasma is defined as a ‘tank’ of partially ionized quasi-neutral gas that contains charged and neutral species such as electrons, positive ions, negative ions, radicals, atoms, and molecules. In this ionized gas there is a balance between the densities of negative and positive particles in terms of both macroscopic volumes and time (Luo and Van Ooij, 2002). Plasma surface modification provides a way of tailoring or designing the surface systematically for various purposes using relatively simple operations and at low cost (Dilsiz, 2000). Plasma technologies have been utilized to improve the surface properties of metallic fibers, glass fibers, carbon fibers, and other organic fibers in many applications (Luo and Van Ooij, 2002). Although the plasma technique can be applied to a wide range of substrates, it is not a useful method for multilayer fabrication.

10.1.3 What is layer-by-layer assembly? Compared with the above traditional strategies for surface modification, the layerby-layer (LbL) self-assembly method is an easy and versatile technique for multilayer deposition (see Fig. 10.1). Although it has become the most popular technique for the fabrication of organized, multilayer, organic–inorganic films in recent years, the pioneering work for the LbL technique can be tracked back to the

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Repeat steps (1)–(4)

(1) Cation

(2) Rinse

Cationic adsorption

(3) Anion

(4) Rinse

Anionic adsorption

Charge neutralization

Charge resaturation

Charge reversal

10.1 Schematic outline of LbL assembly via electrostatic interactions.

suggestive report by Iler and Colloid in 1966. In their research, Iler and Colloid fabricated multilayers by alternating the deposition of positively and negatively charged colloid particles for the first time. They also pointed out that a similar technique could be applied not only to colloid particles, but also to polyvalent ions, surfactants, water-soluble polymers, and even proteins (Iler and Colloid, 1966). Unfortunately, this important work did not become public until it was rediscovered and established by Decher and coworkers (Decher and Hong, 1991a, 1991b; Hong et al., 1993). Since then, a great number of papers on LbL film preparation have been published and various techniques have been used. The LbL approach offers a number of key advantages when compared with other methods for surface modification. One of the most prominent advantages of the LbL assembly is its simplicity and low cost; no special or complicated instruments are needed. The basic LbL assembly could be performed using only beakers and tweezers (Ariga et al., 2007). The variety of the applicable multilayer materials and templating substrates is another most pronounced advantage. Furthermore, the LbL deposition is independent of the size and shape of the substrate, which means that the LbL assembly can be realized not only on planar

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substrates, but also on substrates with different shapes such as spheres and fibers. Multilayer films with desirable layered supramolecular structures and integrated properties may be obtained following incorporation of various building blocks into the multilayer in a designed way (Zhang et al., 2007b). Very diverse charged polymers (Lvov et al., 1993a, Decher et al., 1994, Hoogeveen et al., 1996, Chen and McCarthy, 1997) have been successfully assembled into thin films using the LbL technique, for example: poly(ethyleneimine) (PEI); poly(allylamine) (PAH); poly(diallyldimethylammonium chloride) (PDADMAC); poly(styrenesulfonate) (PSS); poly(vinylsulfate) (PVS); and poly(acrylic acid) (PAA). The LbL technique is not limited to polyelectrolytes, almost any type of macromolecular species can be used, including proteins (Qi et al., 2006); DNA (Ishibashi et al., 2006); viruses (Lvov et al., 1994); inorganic molecular clusters (Keller et al., 1994); nanoparticles (Caruso et al., 1998); nanowires (Tang et al., 2002); organic dyes (Nicol et al., 2003); and polysaccharides (Constantine et al., 2003). A wide variety of substrates have also been used. The two main classes of substrates are planar and colloidal templates (Wang et al., 2008). The interactions in the LbL films are not only based on electrostatic interactions but also on hydrogen bonding, hydrophobic interactions, covalent bonding, and complementary base paring (Decher, 1997). The properties of LbL films – such as composition, thickness, and function – can be readily tuned by simply varying the type of species adsorbed, the number of layers, and the conditions employed in the assembly process. LbL multilayer films, with their well-controlled nano- and micro-scale structures, have received considerable attention in both fundamental studies and applied research. LbL films have been widely explored in areas such as surface modification, drug delivery, electrochemical devices, fuel cells, chemical sensors, nanomechanical sensors, and nano-scale chemical and biological reactors (Zhai et al., 2004a; Zhai et al., 2004b; DeLongchamp and Hammond, 2004; Farhat and Hammond, 2005; Shi et al., 2005).

10.2

The LbL deposition technique

10.2.1 Methods Since the introduction of the LbL technique in 1991, it has rapidly expanded to become a premier technique for the preparation of nano-scale films with designed properties. Recent developments in LbL methods have introduced different driving forces, such as hydrogen-bonding, step-by-step reaction, metal oxide gel films from a surface-gel process, molecular recognition and bio-recognition, charge-transfer interaction, step-wise stereocomplex assembly, electrochemical deposition, inclusion complexes, non-covalent modification, coordination polyelectrolyte, electrostatic complex formation, block copolymer micelles, etc. (Zhang et al., 2007b). However, as an outline of the technique, LbL assembly is mainly

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conducted through electrostatic interaction. As illustrated in Fig. 10.1, the LbL assembly begins with the adsorption of a charged material onto a substrate with opposite charge, which leads to charge neutralization and then charge reversal (Ariga et al., 2007). Further layers are then deposited by the sequential alternate adsorption of the oppositely charged materials onto the substrate until the desired multilayers are reached. Alternation of the surface charge affords a great freedom in the number of layers and layering sequence. Moreover, removal of the templating substrate following LbL film formation can give rise to free-standing nanostructured materials with different morphologies and functions (Wang et al., 2008).

10.2.2 Key principles The structures and qualities of the films produced by LbL assembly substantially depend on a number of parameters, such as the specific multilayer polyelectrolyte and substrate being used, the number of layers assembled, and the experimental conditions employed (including the temperature of adsorption; the ionic strength and pH value of the adsorption and rinse solutions; the immersion, rinsing and drying times for each step, etc.) (Ariga et al., 2007). The electrostatic LbL assembly is performed in an aqueous solution; the polyelectrolyte should therefore be readily water-soluble. The amount of polyelectrolyte deposited onto a surface depends to a great extent on its chain interaction, length, and charge density. The substrate has to carry a minimal surface charge so that the polyion can adhere to it. This charge can be inherent or created by surface modification (Peyratout and Daehne, 2004). Studies on the effect of temperature in LbL assemblies of poly(diallyldimethylammonium chloride) (PDDA)/PSS and PAH/PSS showed that an increase of temperature in the LbL assembly process leads to extension of the exponential growth regime as compared with the linear one. Increases in salt concentration force the polyelectrolytes to contract and form compact globules with reduced net charge, which results in enhanced LbL assembly (Bharadwaj et al., 2006). The pH plays a crucial role in the LbL assembly. Dramatically different polymer adsorption behavior was observed, which was shown by the thickness of adsorbed layers from 8 nm to 0.4 nm over a very narrow pH range (Shiratori and Rubner, 2000). Water rinsing between the consecutive adsorptions was effective for successful alternate adsorption by removing the loosely attached materials. Drying at each step increased the thickness of adsorbed films owing to enhanced surface roughness, which results in low-quality films (Lvov et al., 1999).

10.2.3 Mechanism Although the LbL process only requires simple dipping and rinsing procedures, its assembling mechanism is still not fully understood. Theoretically, the polyelectrolyte association is fundamentally an ion exchange process in which polymer–counterion associations are replaced by polymer–polymer ion pairs. The

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pairing of oppositely charged polyelectrolytes could appear to be driven by strongly exothermic mixing through multiple, cooperative, and specific electrostatic interactions (Ariga et al., 2007). However, it has been revealed recently that the polyelectrolyte association is driven largely by entropy through the release of counterions and waters of hydration from the dissolved polyelectrolyte chains (Bucur et al., 2006). The association constant for complex formation showed little temperature dependence, confirming the ideal nature of complexation. Haynie and coworkers investigated polyelectrolyte complex formation between PSS and PAH using isothermal titration calorimetry and similarly proved the entropy-driven nature of complex formation (Bharadwaj et al., 2006).

10.2.4 Experimental considerations for textile materials The LbL assembly process has been widely utilized to create nano-scale multilayer films on various substrates (Jiang and Tsukruk, 2006; Decher, 1997; Lvov et al., 1993b; Lvov et al., 1996) such as solid planar supports, porous membranes, capsules, nanoporous particles, crystals, and biomimetic structures, however, it has not been extensively employed in textile fibers because textile fibers pose some unique challenges for LbL assembly – including the chemical heterogeneity of their surfaces as well as their irregular shapes (Hyde et al., 2007). Textile fibers often present in the forms of bundles, yarns, or fabrics that have 3D structures with interfiber pores. This means that the substrates are not distributed in the same plane (Ding et al., 2004). The LbL assembly of materials into this 3D structure differs from that observed when using flat substrates because the diffusion speed of the coating material into the 3D structures is different for fibers distributed in various positions. The fibers in the outermost regions have the greatest opportunity of contacting the coating materials and building the LbL films first. The interfiber pores of the LbL-coated fibers will be further reduced after the formation of LbL films on the outermost fibers. The diffusion of coating materials into the 3D structures becomes more difficult due to the reduced spaces between the adjacent fibers. Therefore, the opportunities for the build-up of LbL films in the inside fibers are further decreased, which leads to the irregular deposition of LbL films in the fibrous structures (Ding et al., 2006). However, optimal LbL assembly on textile fibers could be achieved by finely tuning the deposition conditions, for example the concentration of multilayer materials, the number of coating bilayers, and the ionic strength and pH (Ding et al., 2005a). Moreover, with an increase of fiber diameter and interfiber pores or distances, the irregular LbL deposition of multilayer materials could be further eliminated because the fiber’s surface is more like a planar substrate than nano- or submicrofibers (Hyde et al., 2005). Successful deposition of multilayers onto textile fiber surfaces via the LbL technique could open the door to the development of functional textiles for a broad range of applications.

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10.3

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LbL deposition on textile surfaces

10.3.1 Deposition on traditional textile fibers In the last 5 years, the LbL deposition of various materials – such as polyelectrolytes, charged nanoparticles, and non-reactive dyes – onto textile fibers in a controlled manner has been extensively studied and many fascinating results have further confirmed that textile fiber could be an excellent substrate for LbL deposition and that LbL assembly is a very versatile technique for textile surface modification. Cotton fibers were firstly reported to be coated with PSS and PAH through LbL deposition (Hyde et al., 2005). The cotton fibers were initially treated with 2,3epoxypropyltrimethylammonium chloride to positively charge the surface to support the formation of multilayer thin films. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) indicated that uniform multilayers of PSS and PAH were formed on the cotton surface with thicknesses varying from 18 to 22 nm per layer of PAH/PSS. Moreover, the LbL deposition was found to be more dependent on the nature of the polyelectrolytes than that of the substrate (Hyde et al., 2005). The success of LbL assembly on cotton fiber confirmed the feasibility of developing functional textiles for future applications using this versatile technique. In order to further study the effect of surface charge on the growth of PSS/PAH nanolayers on cotton fibers, three different surface charge density levels on cotton fibers were prepared by controlling the ratio of 3-chloro-2-hydroxyl propyl trimethyl ammonium to NaOH in the cationization process. The elemental analysis and XPS data showed that the PSS/PAH deposition was not significantly influenced by the charge densities on the cotton. The uniform build-up of further nanolayers was found to be less sensitive to the charge on the substrate surface once a critical number of layers was reached, usually one to five bilayers (Hyde et al., 2007). Laccase and urease were deposited onto cotton fibers through LbL assembly by alternate adsorption with oppositely charged PDDA which acted as an electrostatic glue between the proteins. The quartz crystal microbalance (QCM), and ζ-potential analysis indicated the regular step-wise formation of organized polyelectrolyte and enzyme multilayer films with thicknesses of 15–20 nm. The catalytic activities of the enzymes in the biocomposites were proportional to the number of deposited enzyme layers. Around 50% of its initial activity was retained after 14 days of storage at 4 ºC in water for laccase–fiber biocomposites. Urease– fiber biocomposites were successfully applied for a biomineralization application to grow spherical calcium carbonate microparticles with a diameter of 1–7 µm that could be used for paper whitening (Xing et al., 2007). LbL deposition of polyelectrolyte multilayers was also utilized to improve the color fastness of dyes in silk after washing (Dubas et al., 2007). The deposited polyelectrolyte multilayers (PEMs) of PDADMAC/PSS effectively prevented the

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release of the dyes inside the silk fiber by acting as an electrostatic barrier. The results showed that the negatively charged PEM with PSS on the outermost layer was more efficient in preventing the negatively charged dye from being released than the positively charged PEM with PDADMAC on the top layer. It was also found that the color fastness was not purely proportional to the number of PDADMAC/PSS layers and that 20 layers of PDADMAC/PAA was the optimum. With a further increase of layer number from 20 to 30, the color fastness of the dyes in the silk was not improved correspondingly (Dubas et al., 2007). LbL deposition is not limited to the above natural textile fibers and can also be applied to synthetic fibers with low charge density. Cationic PDADMAC and anionic Scarlet dye were deposited on nonwoven nylon through LbL assembly. The result that the K/S value at 510 nm increased linearly with the number of PDADMAC/dye layers was correlated with a proportional increase in dye adsorption. The growth of PDADMAC/dye on nylon fibers was highly dependent on the number of layers, salt concentration, and concentration of chemicals, but almost independent of the dipping time. A dipping time as short as 15 s was sufficient for the deposition of the PEM. Dye deposition was enhanced when the dye and PDADMAC concentrations were increased from 0 to 1 mM but was decreased when the PDADMAC concentration was further increased to 50 mM. The optimum salt concentration for PDADMAC/dye film growth was 0.5 M (Dubas et al., 2006b). Sequential deposition of PDADMAC and silver nanoparticles capped with poly(methacrylic acid) (PMA) (PMAcapAg) onto nylon or silk fibers through LbL assembly led to the formation of thin films containing silver nanoparticles which possessed antimicrobial properties. The measured K/S values for both silk and nylon fibers were increased proportionally with the number of deposited layers. The increase in the K/S value of the nylon fibers was found to be significantly lower than that of silk fibers, which indicated the lower deposition rate of silver nanoparticles on nylon fibers. However, the LbL deposition on nylon fibers was not as uniform as on the silk fibers. The deposition of 20 layers of PDADMAC/PMAcapAg onto the fibers resulted in 80% bacteria reduction for the silk fiber and 50% for the nylon fiber (Dubas et al., 2006a). LbL assembly of collagen and chondroitin sulfate (CS) has been successfully conducted on the surface of a polyethylene terephthalate (PET) vascular graft which was first hydrolyzed in concentrated alkaline solution to endow the surfaces with negative charges. The step-wise layer growth of collagen/CS on the PET was monitored by UV–visible spectroscopy and XPS. Stability measurements showed that the multilayers could pass through the endothelial cell’s 8-day culture period. In vitro endothelial cell culture revealed that the incorporated biomacromolecules could significantly improve the cell attachment, proliferation, and viability. SEM showed that cell had a more spreading morphology with preserved phenotype (Liu et al., 2007).

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Syringe pump Collector

High-voltage power source

10.2 Schematic illustration of the nanofiber production process.

10.3.2 Deposition on electrospun nanofibers To further extend its application in textiles, LbL assembly has been combined with the electrospinning method to study the fabrication of functional nanofibers that could potentially be utilized in filters, sensors, catalysts, electrodes, protective clothing, wound dressings, artificial blood vessels, controlled drug delivery, and tissue growth. Electrospinning (see Fig. 10.2) is a straightforward, inexpensive, and efficient method for generating micro- and nano-scaled fibers with high specific surface areas under high voltages (Huang et al., 2003). A fluorescent probe named hydrolyzed poly[2-(3-thienyl) ethanol butoxy carbonyl-methyl urethane] (H-PURET) was deposited onto electrospun cellulose acetate (CA) nanofibers with diameters in the range of 100–400 nm. The fluorescence of these nanofibrous membranes can be quenched by 6.9 × 106 and 3.5 × 106 M for cytochrome c and methyl viologen (MV2+) in aqueous solutions, respectively (Wang et al., 2004). Oppositely charged anatase TiO2 nanoparticles and PAA were alternately deposited on the surface of negatively charged CA nanofibers through electrostatic LbL assembly (see Fig. 10.3). The crystalline phase of anatase TiO2 was retained in the resultant film (see Fig. 10.4). The Brunauer–Emmett–Teller (BET) surface area of the five-bilayer-coated CA nanofibers increased from 2.5 m2/g for uncoated nanofiberss to 6.0 m2/g for coated fibers while the average diameter of the fibers also increased from 344 to 584 nm. This may be due to the rough surface of the deposited multilayers (Ding et al., 2004). The TiO2/PAA-coated CA nanofibers

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(b) 200 nm

10.3 (a) SEM image of LbL-coated fibrous mats; (b) TEM image showing cross-section of film-coated fiber embedded in epoxy resin.

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Intensity

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10.4 Wide-angle X-ray diffraction (WAXD) patterns of (a) the uncoated and (b) the film-coated fibrous mats.

were further modified by fluoroalkylsilane (FAS) to prepare superhydrophobic surfaces (see Fig. 10.5). The rough surface caused by the LbL coating of TiO2/PAA could mimic the nano-sized grooves along the silver ragwort leaf (see Fig. 10.6). Moreover, the rough surface caused by LbL coating could adsorb more FAS than the smooth surface of pure CA and FAS was proved to be the key material for changing the surface from hydrophilic to hydrophobic, and even to superhydrophobic. Thus, the hydrophobicity (see Fig. 10.7) of FAS-modified LbL-coated CA nanofibers increased with the increase of surface roughness (Ogawa et al., 2007). The growth of PAH/PAA films on CA nanofibers through LbL assembly was investigated by regulating the pH and the number of bilayers. It was found that deposition at pH 7.5 for PAH and pH 3.5 for PAA was much quicker than that for PAH and PAA both at pH 5. The average diameter of LbL-coated fibers and the film thickness increased with increasing the number of PEM bilayers. The results of atomic force microscopy (AFM) indicated that the surface of the LbL-coated fiber was rougher than that of the uncoated fiber (Ding et al., 2005a). Polyoxometalate (POM) (H3PMo12O40) was deposited onto electrospun CA nanofibers via an electrostatic LbL assembly with oppositely charged PEI. SEM, Fourier transform infrared (FTIR) spectra, XPS, and wide-angle X-ray diffraction (WAXD) data confirmed the incorporation of H3PMo12O40 in the multilayer films. The morphology of LbL-coated CA nanofibers was studied by controlling the pH and ionic strength of PEI solutions, the number of PEI/H3PMo12O40 bilayers, and

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Cross-section of CA nanofiber (–)

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(2) PAA(–) adsorption

Repeat steps (1) and (2)

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10.5 Schematic diagram illustrating the preparation of super-hydrophobic surfaces via LbL coating and FAS surface modification.

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10.6 SEM images of CA fibrous membranes deposited with various bilayers of TiO2/PAA: (a) 5, (b) 10, (c) 20, and (d) 30 bilayers.

the concentrations of POM. The results showed that the growth of LbL films at pH 2.5 for PEI and H3PMo12O40 was much faster than that for PEI at pH 9 and POM at 2.5. The concentration of H3PMo12O40, the number of bilayers, and the ionic strength of PEI also exhibited a strong influence on the LbL assembly. Moreover, the porous LbL-coated fibers could be fabricated by increasing the ionic strength of the PEI solution by adding NaCl (Ding et al., 2006). Another POM, α(P2W18O62)6– (abbreviated as P2W18), was deposited on a poly(vinyl alcohol) PVA/ indium tin oxide (ITO) electrode with PDDA through LbL assembly. Electrocatalytic effects of the PDDA/P2W18 multilayer-coated PVA/ITO electrode on NO2– were observed (Shan et al., 2007). Polyelectrolyte multilayers composed of PSS/PAH were deposited onto electrospun nylon 6 fibers via electrostatic LbL assembly to enhance their mechanical properties. The morphology of LbL-coated nylon 6 was uniform and smooth under one to five bilayers. It was found that a higher degree of alignment of electrospun nylon 6 fibers and LbL-coated multilayers resulted in higher tensile strength compared to that of the random and pure nylon 6 fibers (Park et al., 2008).

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10.7 (a) Water contact angles and corresponding shapes of water droplets for the FAS-modified CA fibrous membranes deposited with various bilayers of TiO2/PAA. (b) Water-roll angles for the CA fibrous membranes deposited with various bilayers of TiO2/PAA.

Nano-modification using layer-by-layer deposition methods –

Cross-section of CA nanofiber(–)

–

– –

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– – –

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10.8 Schematic diagram illustrating the fabrication of POM nanotubes via LbL coating and thermal removal of the electrospun nanofibers.

10.3.3 Nanofibers as templates for nanotubes or hollow nanofibers The combination of electrospinning and LbL assembly could also be used to fabricate nanotubes or hollow nanofibers by selectively removing the core fiber template through thermal degradation or solvent dissolution. This approach is known as the LbL templating technique (Caruso, 2000). Pure H4SiW12O40 nanotubes with Keggin-type structure were prepared by calcination of CA nanofibers that were LbL-coated with PAH/H4SiW12O40 (see Fig. 10.8). FTIR and WAXD results indicated that the nanotubes only contained pure H4SiW12O40 with Keggin structure after thermal degradation of CA and PAH. SEM and TEM images (see

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1 µm

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10.9 (a) SEM and (b) TEM images of cross-sectional fibers calcined at 380 ºC.

Fig. 10.9) showed that the wall thickness of the nanotubes was 50 nm (Ding et al., 2005b). PAH/PSS, polyA15G15/polyT15C15, and polyelectrolytes/Au nanoparticle hollow fibers were fabricated by dissolving the inner polystyrene (PS) fibers which were initially coated with PAH/PSS, polyA 15G 15/polyT 15C15, and polyelectrolyte/Au nanoparticle multilayer films. These formations of regular

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hollow fibers indicated the homogeneity and integrity of the LbL coating on the PS nanofibers (Mueller et al., 2006). It was found that if the ratio of the thickness of the LbL films (PAH/PSS) to the radius of the template PS is larger than 0.17, the hollow PEM fibers obtained would retain their round tube structure after they were dried. The inner diameter of the hollow fibers (400–500 nm) was much smaller than that of the uncoated PS fibers (920–1320 nm) because the hollow fibers shrunk during the dissolution of core PS fibers (Ge et al., 2007a). The Young’s modulus of the PAH/PSS hollow nanofibers was 21.6 GPa, which was much larger than that for most synthetic organic fibers and similar to that of human bone fibers (Ge et al., 2007b). The hollow multi-walled carbon nanotube (MWCNT)/ polyelectrolyte nanofibers were prepared by selective removal of part of a component in the template, i.e. PS. The existence of the MWCNTs in the hollow nanofibers was confirmed by Raman spectra, TEM, and SEM. MWCNTs could effectively prevent the collapse of the hollow MWCNT/polyelectrolyte nanofibers in air because the mechanical strength of the obtained hollow fibers was enhanced (Pan et al., 2007).

10.3.4 Strengths and weaknesses of LbL for textile surface modification The LbL technique is one of the most simple, low-cost, versatile, and controllable surface modification methods for textile fibers. The wide choice of both materials for LbL films and structures of templating substrates allows the fabrication of higher dimensional structures from functional components. In recent years, considerable effort has been devoted to the controlled fabrication of LbL films on fibers through tuning the concentrations of the multilayer material, the number of coating bilayers, the ionic strength and pH of the solution, and the dipping and rinsing time, etc. However, some challenges still remain in the application of the LbL technique to textile fibers; for example, the poor stability of LbL films, the irregular growth of LbL films, and the limited number of bilayers that can be coated onto textile fibers. During the laundry process, the electrostatic interactions could be destroyed and the LbL films could be peeled off from the textile fibers resulting in loss of functionality. Moreover, some previous reports have shown that the LbL growth of multilayer films was greatly influenced by the substrates before it reached a critical number of bilayers. For example, the variation in layer growth during the deposition of one to five bilayers could be very large; this will greatly affect the functionality of the LbL films (Hyde et al., 2007). Additionally, the electrospun nanofibrous mat lost its porous structure (see Fig. 10.6) when nanofibers were used as a template for LbL deposition over 20 bilayers (Ding et al., 2004; Ogawa et al., 2007).

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Conclusions and future trends

The LbL self-assembly technique, a unique nanotechnology, has been applied as an efficient nanocoating process for the deposition of functional materials on traditional textile fibers and nanofibers. Fundamental research results of previous reports indicate that the principles of coating LbL-structured, multilayer nanofilms on fibrous structures have been well established. This now permits subsequent researchers to explore the many up-coming applications for these functional textile fibers – such as protective clothing for chemical weapons, self-cleaning superhydrophobic textiles, toxic gas filters using physical adsorption or chemical decomposition, highly sensitive coatings for QCM nanofiber sensors, electronic devices, anti-electrostatic textiles, anti-microbial textiles, color fastness agents, biocatalytic systems, electrodes, biocomposites, etc. Current LbL assembly research in textiles has mainly focused on the use of synthetic, charged polymers for the assembly of LbL films using electrostatic force. In fact, the driving force of LbL assembly is not limited to electrostatic interactions, and has been expanded to include various kinds of physico-chemical interactions. For example, LbL assembly based on metal–ligand interaction or metal coordination has a long research history (Lee et al., 1988). The introduction of covalent bonding to the LbL technique would significantly strengthen the stability of the assembly structure on textile fibers. Various other interactions, including biochemical recognition and binding of the substrates with incredibly high efficiency and selectivity, could be utilized as a driving force for LbL assembly. To date, adhesion between the deposited LbL layers and the substrate has been achieved using conventional driving forces – including electrostatic interactions, hydrogen bonding, step-by-step reactions, sol-gel processes, molecular recognition, charge-transfer, step-wise stereocomplex assembly, and electrochemistry – and unconventional methods such as inclusion complexes, non-covalent modification, coordination polyelectrolytes, electrostatic complex formation, and block copolymer micelles. The LbL formation process could be monitored and characterized according to surface charges, structures, sizes, chemical compositions, and bonding formations. The strength of the interactions between the formed layers and substrates could easily be assessed by measuring the release of the adsorbed components under actual application conditions. Additionally, the instruments involved in LbL assembly could be improved so that the process is controlled automatically. For example, a solid plate was attached to the arm of a robot together with a QCM as the sensitive mass detector, this provided frequency shifts during adsorption of the material to control the thickness of LbL films (Shiratori and Yamada, 2000). Moreover, the LbL method could be combined with other techniques such as spin-coating (Lee et al., 2001), electrospray (Schlenoff et al., 2000), and etching processes (Li et al., 2006) to extend its practical applications.

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Acknowledgements

All the authors of the cited papers and the data adapted for this chapter are gratefully acknowledged.

10.6

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11 Surface modification of textiles for composite and filtration applications A. S. HOCKENBERGER

Uludag University, Turkey

Abstract: Fiber-reinforced composites and textile filters are being increasingly used in our daily lives. Their performances strongly depend on the fibers’ surface properties. A high-performance fiber gives its strength to a composite through the interface between the matrix and the fiber surface. The particles to be filtered are also collected on the fiber surface. However, in some cases fiber surface characteristics are not very suitable for good adhesion or high rates of particle collection. There are several methods for surface modification of fibers. In this chapter the surface properties of reinforcing and filtering fibers are described and the surface modifications for special applications are explained. Key words: fiber-reinforced composites, surface modification, filtration, filtration fibers.

11.1

Introduction

Composite materials appeared very early in human technology. One of the earliest known composites is a brick in which straw is mixed with mud or clay. The straw allows the water to evaporate and distributes cracks in the clay uniformly and improves the strength of the brick. Today’s composites are highly structural materials with high modulus of elasticity. These highly engineered composites provide materials with stiffness, high strength and low weight features. Highperformance fibers with excellent mechanical, thermal and chemical properties have introduced a new generation of composites. Composites can now be used in many different areas such as biomedical, construction, automotive, defense and aerospace applications. The mechanical properties of a fiber-reinforced composite depend mainly on the degree of adhesion between the fiber and the matrix. However, most of these reinforcing fibers do not have the required surface properties for proper adhesion. In this chapter some of these fibers are described and surface modification methods are analyzed with application examples given. Filtration is a separation process. From very early days people tried to protect themselves from dust by putting a piece of fabric around the nose and mouth. As knowledge and understanding of air and air-borne contaminants, and their effect on people, have increased, filtration has become more important. Filtration gives 238

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healthier and cleaner products and environment. Air and water filtration are very important and textile structures are widely used for the filtration of air and liquids. Textile structures provide a three-dimensional network of fibers and high filtration efficiencies; filtration efficiency depends on the particle collection rate. Textile fiber surfaces collect the particles and therefore fiber surface properties are crucial to the filtration performance. In the second part of the chapter filtration fibers are explained and the effect of their surface properties on filtration is elucidated.

11.2

Surface modification of textiles for composites

11.2.1 Basics Adhesion The mechanical properties of a fiber-reinforced composite depend mainly on the degree of adhesion between the fiber and the matrix. When a material interacts with another or with the surrounding environment, it is the nature of the surface field of forces that determines the kind of interaction (Garbassi et al., 1994). Several theories have been introduced to provide an explanation for adhesion phenomena. However, no single theory explains adhesion in a general, comprehensive way. Presently four main mechanisms survive, involving physical or chemical forces. These are: mechanical interlocking, interdiffusion, electrostatic interaction and chemical interactions. These four mechanisms work at different scales of distance between the two materials. Mechanical interlocking occurs when an adhesive penetrates into the irregularities (such as pores, holes) of the adhered surface of a substrate, and results in mechanical entanglement (Jennings, 1972). Textile surfaces are considered to be good examples of mechanical interlocking as they have a porous nature. The surface treatments that result in microroughness on the surface improve adhesion strength by proving mechanical interlocking. The roughening of the surface can also result in the formation of a larger surface. In the interdiffusion theory, some of the long-chain molecules or at least some segments of the two surfaces reciprocally diffuse into each other. This theory requires that both surfaces are polymers and are above the glass transition temperature. Therefore the theory is not applicable for polymer–metal systems and is of limited use in highly cross-linked and crystalline structures. Electrostatic attraction theory is based on the difference in the electronegativities of the surfaces. Electrons run from one surface to another. The theory has been the subject of some controversy, for example the fact that the electrostatic double layer can be identified only after separating the adhesive bond and its effect on the adhesion strength is considered to be magnified (Garbassi et al., 1994). Chemical interactions between two surfaces require a variety of forces, generally considered

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as van der Waals forces, such as non-polar dispersion forces, dipole–dipole interactions, dipole-induced dipole interactions. This theory is the most important mechanism in achieving a good adhesion. Many polymers have very good bulk properties and are inexpensive; however, they are inert and have a low surface energy. Many industrial applications such as composites require these polymers to have special surface properties. Therefore, methods that enable surface modification of polymers without affecting the bulk properties have received a great deal of attention. Polymer surface modification treatments change the roughness, chemistry and wettability of a surface and affect the surface energy. Surface energy The basic concept of surface energy is that it is the excess energy associated with the presence of a surface. It is a thermodynamic parameter that controls the surface composition of materials (Packham, 2003). G = G0 + ψ where G is the fracture energy, G0 is the surface energy and ψ represents other energy absorbing processes (i.e. plastic deformation during fracture). An increase in G0 will result in a large increase in adhesion. Surface activation increases the surface energy by creating chemical bonding between the surfaces. High surface energy results in better adhesion. Surface roughness also affects adhesion and often increases the adhesion. The surface energy term G0 is the surface excess energy per unit area, therefore G0 = ∆G/A Introducing chemically active groups onto the surface results in an increase in ∆G. ∆G may also be increased as a result of roughening the surface. Adhesion to rough surfaces may be effective because of the intrinsically high surface energy of atoms on an asperity surface. Rough surfaces may also redistribute the stress so as to increase energy dissipation (Packham, 2003). The contact angle measurement is the most common method of solid surface tension measurement. When a drop of liquid is placed on a solid, the surface tension of the liquid is larger than the surface tension of the solid, it makes a definite angle of contact between the liquid and the solid faces. When the surface tension of a solid increases, the contact angle (θ) decreases. Total wetting (θ = 0) occurs when surface tension of the liquid is smaller than the surface tension of the solid. Therefore when the surface tension of the polymer is increased, wetting of the surface is also increased (see Fig. 11.1) (Garbassi et al., 1994).

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Wetting decreases

θ

θ

θ

Solid

Contact angle inceases

11.1 Contact angle measurements.

11.2.2 Composite structure A composite material is a microscopic combination of two or more distinct materials, having a recognizable interface between them (Miracle and Donaldson, 2006). Most composites consist of a bulk material (matrix), and a reinforcement material added primarily to increase the strength and the stiffness of the matrix. This reinforcement is usually in fiber form. Composites are often classified according to the matrix constituents, i.e. (a) polymer matrix composites – these are the most common type, a polymerbased matrix is used with a variety of fibers used as the reinforcement; (b) metal matrix composites – a metal such as aluminum is used as the matrix, mostly used in the automotive industry; (c) ceramic matrix composites – these materials use a ceramic as the matrix and are used in very high temperature conditions. Fibers, particles, flat flakes and fillers can be used as reinforcement materials. Fiber-reinforced materials can also be subdivided into those with short fiber reinforcements or those with long fiber reinforcement. Polymer matrix composites are the most common form of composites and will be discussed here. Fiber-reinforced composites represent a class of composite materials that combine low weight with good mechanical properties. In a composite material, the fibers are surrounded by a thin layer of matrix. The matrix protects the fiber from the environment and holds the fiber in the required orientation. The reinforcing fibers are responsible for the strength and stiffness. The fibers are much stronger and stiffer than the matrix. The relationship between strength and modulus of the composite and the corresponding properties of the fiber and the matrix depends on the fiber length, fiber orientation, fiber surface, fiber cross-section and fiber linear density.

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The reinforcement material and matrix must be compatible for good adhesion. Thermoset resins, thermoplastic resins and rubber are used as matrix materials. Thermoset matrices have low viscosity and surface wetting; however, they show poor strength and poor impact performance. Polyester and epoxy are the most widely used thermoset matrices. Thermoplastic matrices include polyethylene, polypropylene, polystyrene, PEEK (polyetheretherketone), PPS (polyphenylenesulfide) and PEI (polyetherimide). They give (form) composites that soften on heating. Rubber is also widely reinforced with fibers or steel wire to improve its strength, stiffness and durability. Tires, conveyor belts and hoses are the most widely used applications of fiber-reinforced rubber composites (Yang, 1972). The matrix provides a solid form to the composite and transfers load to the reinforcement fibers. The overall properties of the composite structure depend on the properties of the individual components and the adhesion between them. High adhesion results in high mechanical strength of the composite. When adhesion between phases is modified, materials with a range of properties and applications can be developed. The properties of the composites are determined by: • • • • •

the properties of the reinforcing fibers; the properties of the matrix; the ratio of fiber to matrix; the geometry of the reinforcing fibers and their orientation in the matrix; the properties of the interfacial adhesion.

Fiber properties include: fiber length, fiber orientation, fiber surface, fiber crosssection, fiber linear density. Composites made with higher linear density isotropic fibers, such as boron, usually have a much higher compressive strength than tensile strength. However, isotropic fibers with lower linear densities, such as glass, are usually stronger in compression than in tension; whereas composites made with small-diameter anisotropic fibers, such as aramids, are usually stronger in tension with a low compressive strength. Very short fibers act as stress concentrators; on the other hand, very long fibers are difficult to process and are damaged during molding. Very thin and continuous fibers are mostly preferred for composite applications. Woven, braided, knitted, laminated, nonwoven fabric structures are also used to reinforce composites. Two-dimensional woven fabrics are anisotropic and have less modulus fiber reinforcement due to crimp interchange. Woven fabrics can be produced from one single fiber type or different types of fibers. Fabric construction and fabric density are the most important properties for composite applications. High density prevents matrix penetration and gives rigid structures; on the other hand, loose fabrics do not give the required support to the matrix. Knitted fabrics are mostly used for flexible composites. Nonwovens are also used for composites but as there is no yarn interlacing, they are not strong enough to provide proper support (Adanur, 1995).

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11.2.3 Tire cord technology Structure of a tire A typical tire carries load and transports it at the desired speed. Some secondary functions of a tire include the transmission of driving torque, providing cornering ability, vibration damping, rolling ability over snow or ice, thermal and cutting resistance, dimensional stability, safety and durability. Because of these functional requirements, tires are constructed as composites (Yang, 1972). A tire is a textile– steel–rubber composite. The steel and textile cords reinforce the rubber and are the primary load-carrying structures within the tire. Tires have plies of reinforcing cords extending transversely from bead to bead, on top of which a belt is located below the tread. The belt cords have low extensibility and are made of steel and fabric depending on the tire application. Cotton, rayon, nylon, polyester, steel, fiberglass and aramid fibers are suitable for tire applications because of the performance demands of fatigue resistance, tensile strength, durability and resilience (Kovac and Rodgers, 1994). A tire is an assembly of a series of parts or subassemblies, each of which has a specific function in the service and performance of the product. Figure 11.2 shows the key components of a highperformance passenger tire. The key needs of a tire system are impact and fatigue resistance, tire life and durability, tread life, cornering and handling capacity, and high-speed performance. The required reinforcing fiber properties are high strength and modulus, strength retention after fatigue, and adhesion to rubber (Tanner et al., 1989).

8 7 6

5 2

3

1 4

11.2 Key tire components (www.wbcsd.org).

1 2 3 4 5 6 7 8

Inner liner Radial body plies Bead filler Bead Sidewall Steel belts Nylon overlay Tread

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11.3 Polyethylene naphthalate (PEN) cord (twist 530 T/m).

Tire cord construction In fiber-reinforced composites, fibers are usually evenly distributed through the matrix. However, in tire applications, fibers must be in the yarn form and the yarns must be twisted (see Fig. 11.3) and processed into cords to be able to reinforce the rubber and provide desirable composite mechanical properties for tire performance. First, the yarn is twisted on itself to give a defined number of turns per inch then two or more twisted yarns are then twisted into a cord. Generally the direction of twist is opposite to that of the yarn, this is termed a ‘balance twist’. In this way, the fiber is also placed as parallel as possible to the cord axis. This is very important in terms of fatigue performance. There are several reasons for twist in a tire cord (Kovac and Rodgers, 1994): (a) twist imparts durability and fatigue resistance to the cord, though tensile strength can be reduced; (b) without twist, the compressive forces would cause the cord outer filaments to buckle; (c) increasing twist in a cord further reduces filament buckling by increasing the extensibility of the bundle. Cord–rubber adhesion The performance of cord–rubber composites is strongly dependent on the development of adhesion between reinforcing fibers and rubber. A good adhesion improves the mechanical performance of the tire composite (see Fig. 11.4). There are very significant differences between fibers and rubber from a chemical and mechanical

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11.4 Cord layers in a tractor tire.

point of view. The polarity and the high modulus of fibers are very different from the non-polar nature and low modulus of rubber. The development of adhesion between fiber and rubber can occur through mechanical, chemical and molecular interactions. Therefore, in order to understand the adhesion mechanism one should examine the structure and properties of the upper layers of the fibers. The surface energy and macromolecular mobility affect the characteristics of adhesion. Diffusion, adsorption and interlocking are the essential mechanisms of the adhesion process (Chawala, 1994). Molecular bonding is achieved through molecular interdiffusion between adhesive and substrate (fiber and rubber) and hydrogen bonding. The resorcinol– formaldehyde–latex (RFL) adhesive system is used in the tire industry. The latex component makes the adhesive layer flexible and is mixed with a rubber layer through a secondary bond and co-vulcanization. The RF components react with the bonding group in the fiber. In dynamic conditions, stresses acting on the rubber matrix are transmitted to the fiber across the interface. Therefore, stronger interfacial adhesion is needed compared with conventional reinforcing. Owing to the different surface properties of the fibers, the selection of adhesive systems is crucial in order to achieve better adhesion between rubber and fiber. Flexible and heat- and fatigue-resistant adhesives are necessary to meet the requirements of cord–rubber composites for industrial applications (Chawala, 1994). Some adhesive systems, such as RFL, work for polyamides, owing to their chemically active surface; the RFL system does not provide good adhesion for polyester because of lack of functional groups and the development of adhesion through hydrogen bonding in aramid is much more difficult than in nylon. Therefore, both polyester and aramid fibers are treated with a pre-dip before being treated with a standard RFL dip.

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Surface properties of reinforcing fibers and applications

Polymers have been applied successfully in many fields; composites are one of these fields. In general, special surface properties – such as chemical composition, hydrophilicity, roughness, crystallinity, conductivity, lubricity and cross-linking density – are required for the success of these applications. Polymers often do not have the required surface properties for composite applications. However, they are easy to process, are inexpensive and show a wide variety of chemical and mechanical performance. Therefore, much research work has focused on developing surface treatments to change the chemical and physical properties of polymer surfaces without affecting bulk properties (Chan, 1994). Techniques for modifying polymer surfaces enhance the chemical nature of the surface of the polymers. These methods: produce special functional groups at the surface; increase surface energy, electrical conductivity, hydrophilicity or hydrophobicity; improve chemical inertness, dyeability and handling; or modify surface crystallinity or roughness. Fiber-reinforced composites are used in many areas. In a composite material the fibers are surrounded by a thin layer of matrix material that holds the fibers in the desired orientation. The strength of the adhesion between the fiber and the matrix strongly depends on the surface properties of the fiber. In order to control the properties of the interface, surface treatments are often used on the fiber.

11.3.1 Polyesters The term ‘polyester’ is used to designate a polymer that has an ester group in its main chain. In the textile industry the term ‘polyester fibers’ is generally used to describe fibers made of polyethylene terephthalate (PET). PET is the most common fiber-forming polyester. The structure of the fibers depends on the fiberforming process (spinning, drawing, heat setting). Therefore, the properties of the PET fibers differ depending on their processing conditions and thermal history. In addition to PET, other polyester fibers have been developed and are used in many other areas such as in the carpet industry. The majority of research related to the surface structure, modification and characterization of polyester fibers is on PET. Conventional PET fibers, dimensionally stable PET (DSPET) fibers and high-tenacity PET fibers are widely used in composites and in the tire industry. PET fibers have many desirable properties; such as relatively high tenacity, low creep, good resistance to strain and deformation, high glass transition temperature, good resistance to chemicals, relatively low price, they are easy to obtain due to high production tonnages in recent years and adjustable fiber-forming process parameters can produce PET fibers with different structures (Hsieh, 2001). The hydrophobic and oleophilic nature of PET fibers lead to poor adhesion to rubber and plastics. Surface characteristics such as liquid wetting, liquid

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O [

C

C.O.CH2.CH2.O

]n

O

1.5 PET structure.

repellency, soil release, adhesion, friction, handling, light reflection, and static properties are important with regard to the processing and performance properties of fibrous products (Chan, 1994). PET (Fig. 11.5) is a step polymerization product of ethylene glycol and terephthalic acid (or dimethyl terephthalate). The alkyl segment of the monomer unit (i.e. the methylene sequence) is derived from the glycol. The chemical unit consisting of the aromatic ring with its two associated carboxyl groups is known as the terephthalate residue and derives from terephthalic acid. PET fibers have a microfibrillar structure containing crystalline and noncrystalline regions. The surface structure of the fiber is affected by fiber size, cross-sectional shape, spinning speed, drawing rate, thermal treatments, chemical structure, crystallinity and orientation, the type of end groups and the liquid wetting. The surface composition of fibers is often different from the bulk. For example, assuming minimum end-group effects due to the high molecular weight of PET, the carbon-to-oxygen (C/O) ratio in the repeating unit of the PET structure should be 1.875; however, the C/O ratio of polyester fibers was reported to be 2.92 (Wakida et al., 1993). This is much higher than the 1.875 C/O ratio of the bulk. This indicates that only 64% of the oxygen along the PET chain is located on the fiber surface. Therefore the polar end groups (–OH and –COOH) and ester bonds along the chain are away from the surface. The more carbon-rich surface of the PET gives lower surface energies. The lower surface oxygen content of the PET fibers also suggests that surface chain segments are largely in the non-crystalline region. The fiber PET surface also has a hydrophobic nature. However, due to polar end groups and the ester groups along the PET chains, PET shows a more polar nature than expected (Hsieh, 2001). The fineness and cross-sectional shape of fibers also have an effect on the surface properties. The fineness of the fiber is described in grams per 1000 m or 9000 m length of fiber. Smaller fiber diameters give a higher surface-to-volume ratio. Increasing the shape factor or the irregularity of the fiber cross-section is another way of increasing surface-to-volume ratio. The higher surface-to-volume ratio changes some fiber properties, such as dyeability, appearance, thermal insulation, moisture, liquid transport, adhesion to matrix in composites and luster. In composites, the reinforcement fibers bear almost all of the applied load and the matrix transfers the load through the interface. Therefore fiber strength and the properties of the fiber–matrix interface control the mechanical performance of composites. The chemical inertness, low surface energy, non-polar and

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hydrophobic nature of PET fibers produce an insufficient bond with the polymer matrix. Many surface modification treatments are used with these fibers to enhance the interfacial shear strength between fibers and matrix, to improve the mechanical performance of the composites. Techniques used to modify the PET surface include: treatments with solvents and acidic or basic solutions; mechanical abrasion; graft reactions; plasma treatment; and ultraviolet (UV) excimer laser. Plasma surface modification gives satisfactory results and is environmentally friendly. The surface of the material may be roughened and/or its surface chemical character can be changed. Various gaseous media have been used for PET modification using plasma treatment. Treatment with cold CO2 plasma increased the surface energy of PET fibers (Cioffi et al., 2003). Oxygen plasma-treated PET fibers showed an increase in surface energies from 6.4 × 10–6 to 8.3 × 10–6 J and a decrease in contact angle values from 47º to 13º for treatment times of 5–100 s respectively. The average ultimate tensile strengths of oxygen plasma-treated fiber composites were higher in comparison to the untreated fibers. Oxygen plasma treatment introduced C–O–, C=O and COO– groups onto the PET surface. Wong et al. (2003) studied the surface structuring of PET using a UV excimer laser. They concluded that with the appropriate laser treatment, the hydrophobicity of polyester could be enhanced. The contact angle value was greatly increased with laser treatment. High-performance, advanced PET fibers are superior to conventional rubber reinforcing materials because they show very high strength, dimensional stability, toughness and fatigue resistance, as well as a lower specific weight. In the standard method for the improvement of adhesion strength between the reinforcing polymer fibers and the rubber matrix, the fiber surfaces are treated with the RFL system. This method is not satisfactory for use with polyester owing to the hydrophobic nature of its surface. As a result, a number of adhesives for bonding polyester fibers to rubber have been developed (Chawala, 1994). Several subcoat dips based on the chemical activation of polyester surfaces have been patented. These systems are two-dip systems, a subcoat followed by a second RFL dip; however, for economic reasons, a single-dip system is required. Canadian Industries Ltd developed a single-dip system called N-3. The N-3/RFL-dipped polyester cord provides excellent adhesion to rubber. ICF also developed a singledip adhesive system (Belgian Patent 688424) called Pexul (or H-T). Another way to improve the cord–rubber adhesion is to activate the surface of polyester filaments by applying a finish solution during the fiber spinning process. There are several patents describing polyester fiber surface activation processes. The modification of the PET fiber surface by grafting or plasma treatment to improve adhesion has been widely studied. Krump et al. (2006) studied the adhesion between plasma-treated polyester fibers and a rubber matrix. The PET fibers were treated with different types of plasma reactor (atmospheric plasma and microwave plasma). Exposure of the PET fiber to atmospheric plasma discharge in

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the presence of nitrogen resulted in a significant increase in adhesion with the rubber matrix. Application of pure argon as the plasma gas also increased the interfacial strength between the cord and the matrix. Argon plasma slightly affected the morphology; however, more pronounced changes were observed when nitrogen was used as the plasma gas. Hockenberger and Koral (2007) studied an air-jet texturing system to improve the adhesion of polyester fibers to rubber. The air-jet texturing process produces spun-like yarns by modifying the uniform arrangement of the synthetic continuous multi-filament yarns. Disturbance of the originally parallel arrangement of the filaments to the yarn axis and formation of surface loops anchored in the yarn core, adversely affect the mechanical and fatigue properties, and also the adhesion behavior of the air-jet-textured yarn. Air-jet texturing is an inexpensive and fully mechanical process, with great potential to lead to the development of fiberreinforced composites with good adhesion properties. Although no clear change in the surface topography of single fibers was observed by scanning electron microscopy (SEM) as a result of air-jet texturing, some surface changes were seen using both atomic force microscopy (AFM) and dynamic contact angle measurements. These surface changes, however, were minimal owing to low overfeed levels. Therefore the improvement of the adhesion behavior of conventional PET yarns was mainly attributed to changes in the cross-sectional shape of the yarns after air-jet texturing. Wei et al. (2007) studied the dynamic wetting behavior of plasma-treated PET fibers. PET fibers were treated in oxygen plasma to improve surface wettability. The fiber surface was roughened and functionalized. This resulted in a decrease in advancing and receding contact angles. Aytac et al. (2007) studied the effect of gamma radiation on the properties of polyamide66 (PA66) and PET tire cords. Radiation processing of rubber formulations in the tire industry offers many advantages such as high processing rates, energy savings, etc. The results showed that gamma radiation of tire textile cords in air had a slight effect on some of the mechanical properties of Nylon 66 cords, but had no effect on the mechanical properties of PET cords. This was attributed to the higher radiation resistance of PET. Jamshidi et al. (2005) investigated the cord–rubber interface at elevated temperatures using the H-pull test method; they used Nylon 66, Nylon 6 and PET cords. In all the cord–rubber systems, an increase in test temperature caused a decrease in adhesion. PET fiber cords were modified with argon, oxygen and successive argon/ oxygen cold plasmas. Plasma-treated cords were coated with RFL. The adhesion strength was measured by peel tests. An increased adhesion of 280% was obtained with 30 min argon plasma followed by 30 min oxygen plasma at 75 W power and 40 Pa pressure, without altering the strength of the fibers (Carlotti and Mas, 1998). Ivan et al. (2006) studied different types of surface modification of polyester cords using low-temperature plasma at atmospheric pressure. The first type of cord was activated by pulse surface positive corona discharge generated in a plasma

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reactor or by coplanar dielectric surface barrier discharge in nitrogen or ambient air plasma at atmospheric pressure. The second type of plasma treatment involved the modification of the cords by plasma polymerization in a mixture of nitrogen with butadiene. The highest roughness value was obtained with nitrogen plasma. The butadiene–nitrogen gas mixture resulted in a new polymer layer on the cord surface. Both methods improved the adhesion without having a significant effect on the mechanical properties. The surface free energies of PET fibers with different draw ratios were determined by contact angle measurement (Okamura et al., 1996). The dispersive component of the surface free energy increased with increasing draw ratio owing to the addition of functional groups to the fiber surface as a result of heat treatment; whereas the non-dispersive component remained almost constant. Higher draw ratios orient the molecules in the amorphous regions parallel to the fiber axis. The UV-laser-induced surface modification of PET fibers increased the amorphous PET surface, lowered the ratio of O atoms to C atoms, and CHO groups were also formed (Watanabe, 1998). Coatings of plasma-polymerized pyrrole or acetylene were deposited on aramid fibers, aramid cords and polyester cords. Plasma polymer films improved the pull-out force of fibers and cords embedded in polymers. This was attributed to the interpenetrating network formed between the matrix and the plasma polymer. Various functional groups also contributed to this improvement (van Ooij et al., 1999).

11.3.2 High-modulus polyethylene fibers Fiber-reinforced composite materials consist of high-strength, high-stiffness fibers. Gel-spun polyethylene fibers are ultra-strong, high-modulus fibers that are based on the simple and flexible polyethylene molecule. Ultra-high-molecular-weight polyethylene (UHMWPE) fibers have a high specific strength, high specific modulus and outstanding toughness. However, their poor adhesive properties have limited their use for composites. Therefore their surface properties have been greatly studied and improved. High-performance polyethylene fibers are produced from polyethylene with a very high molecular weight. Gel-spun material is then drawn to a very great extent in order to produce a fiber with very good mechanical characteristics. Meltspinning and drawing is only possible for low-molecular-weight, high-density polyethylene. High-performance polyethylene fibers have a density of 0.97– 0.98 g/cm3. The tenacity is 10–15 times that of good quality steel and the modulus is second only to that of special carbon fiber grades and high-modulus poly(pphenylene-2,6-benzobisoxazole (PBO). Elongation is relatively low, but owing to the high tenacity, the energy required to break the fiber is high. The most important properties of these fibers, therefore, are high strength and high modulus in combination with the low density. Owing to high molecular alignment along the fiber axis, the mechanical properties are highly anisotropic. In the transverse

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direction the mechanical performance is much lower than that in the fiber direction. They are also prone to creep, the deformation increases with loading time. High-performance polyethylene fibers show good flexural fatigue as they are flexible. Polyethylene is hydrophobic and does not absorb any water, it also shows high resistance to acids and alkalis; it is regarded as biologically inert. However, polyethylene has a melting point of around 140 ºC. As high-performance polyethylene fibers have a high energy absorption they are widely used in composites for ballistic protection (Van Dingenen, 2001). Polyethylene surfaces have an extremely low surface energy, ~ 340 mNm, as dispersion forces of polyethylene have the total contribution to the surface energy. Polyethylene fiber will not be wetted by polar resins and will have no intrinsic reactivity with epoxies, unsaturated polyester or vinyl ester resins (Van Dingenen, 2001). Ladizesky and Ward (1983) first recognized the problems of the low surface energy and chemical inertness of UHMWPE fibers in fabricating composites with epoxy resins. They used surface oxidation methods to modify the surfaces of monofilaments. Two methods were compared: chromic acid oxidation and direct oxygen plasma. The oxygen plasma treatment produced a cellular surface on the monofilament with pits. However, a 50% decrease in the ultimate tensile strength of the single filaments was recorded. No surface pitting occurred following the chromic acid treatment. For UHMWPE gel-spun fibers, the oxygen plasma treatment produced a decrease in the contact angle between the epoxy resin and the fibers. However, this was attributed to the removal of the low-molecular-weight material. Chappell et al. (1991) carried out a comprehensive study of the adhesion of UHMWPE to epoxy resins using oxygen and ammonia plasma treatment. Ammonia plasma treatment resulted in the incorporation of amine functional groups onto the fiber surface. A pronounced increase in interlaminar shear strength over composites made from untreated, corona-treated or oxygen plasma-treated fibers was recorded. They also showed that fibers modified by oxygen plasma contain a significant concentration of carbon–oxygen functionalities. Oxidative (oxygen and air) RF-plasma treatment of high-density polyethylene was found to be an effective tool for improving wettability, as well as for increasing its surface microhardness. An increased negative surface charge of plasma-treated polyethylene confirmed the presence of functional ionogenic groups containing oxygen. The vigorous increase of the surface roughness was confirmed by the successful plasma etching (Lehocky et al., 2003) Taboudoucht et al. (2004) investigated the effect of fuming nitric acid (FNA) treatment on the adhesion of ultra-high-modulus polyethylene fabrics to an epoxy resin and the results showed an increase in interlaminar shear strength (ILSS) of a maximum factor of 1.5. Surface energies of polyethylene films were improved by applying atmospheric pressure air glow discharge with aqueous electrolyte cathode onto the surface.

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Distilled water and aqueous solutions of KCl and HCl were utilized as a cathode (Choi et al., 2006). OH groups, C=O groups in ester, ketone and carboxyl groups, and C=O groups in unsaturated ketones and aldehydes were observed on the surface. This modification process improved the surface free energy of polyethylene. Holmes and Schwartz (1990) studied the effect of ammonia plasma in aminating the polyethylene surface. Ammonia plasmas were successfully used to increase the surface energy and adhesive bonding between UHMWPE fibers and polymethyl methacrylate. Rhee et al. (2005) studied the surface modification of fibers by Ar+ ion irradiation under an oxygen environment. The optimal Ar+ ion dose was determined by means of tensile modulus and strength of the polyethylene fiber– vinylester composite as a function of ion dose. An increase in adhesion between polyethylene fibers and vinylester resin was obtained and attributed to C=O groups on the surface. UHMWPE fiber was treated with oxygen plasma and silane coupling agent to improve the interfacial adhesion between the polyethylene fiber and vinylester resin (Moon and Jang, 1997). Micro-pits were formed on the fiber surface during plasma treatment and the mechanical interlocking between the matrix resin and these micro-pits played a major role in improving the interfacial adhesion between the fiber and the matrix. Interface shear stress of the polyethylene fibers was greatly increased by the grafting of pentaerythritol or diethylene triamine onto the HMWPE fiber surface. In this study (Teng and Yu, 2005), polyethylene fibers were oxidized via chemical reactions in an acidic medium, and the carboxyl group was transferred into the acryl chloride and then reacted with pentaerythritol or diethylene triamine to graft the multifunctional group compounds on the fiber surface. The polar functional groups – including –COOH, –OH, and –NH2 – were introduced on the surface. The polar groups improved the wettability. Li and Netravali (2003) studied the modification of high-strength polyethylene fibers through allylamine plasma deposition on the fiber and fiber–epoxy interface. They showed an increase in interfacial shear strength (IFSS) of a factor of 2–3 after allylamine plasma treatments. SEM pictures showed that pull-out failure occurred at the interface, as evidenced from clean fiber surfaces. Ultra-high-strength polyethylene fibers were treated with a pulsed XeCl (308 nm) UV excimer laser to improve their adhesion to epoxy (Song and Netravali, 1998). The X-ray photoelectron spectroscopy (XPS) data indicated the incorporation of both oxygen and nitrogen on the fiber surface resulting in a more polar surface. The SEM photomicrographs showed an increase in surface roughness. All these factors resulted in an increase in IFSS of 200–400%. Intense pulsed high-power ion beam treatment was applied to ultra-highstrength polyethylene fibers (SpectraTM 1000) to improve adhesion to epoxy resins. Chemical and topographical changes of the fiber surfaces were

Surface modification for composite and filtration applications .. ..

.. ..

C

C

O

..

..

N

N

H

H

253

O

11.6 Aramid fibre structure.

characterized using different techniques (Netravali et al., 1999). The surface roughness and polar nature of the fiber surface were increased after the treatment. The IFSS of polyethylene fibers with epoxy resin was significantly improved. This was attributed to increased roughness of the fiber surface, increased interface area, increased polar nature and an improvement in the acid–base component of the surface energy. Polyethylene fibers have a smooth surface with the presence of an outer layer. This is a weak boundary layer and is rich in ether and/or hydroxyl oxygen. Beneath this outer layer is a fibrillar structure. An increase in surface roughness enhanced the adhesion of the fiber to any matrix. Chromic acid, the strongest etchant studied, was used to modify a UHMWPE fiber surface. Chromic acid treatment produced a rough and oxidized polyethylene surface with both ether and carbonyl oxygen resulting in improved adhesion. However, neither hydrogen peroxide nor potassium permanganate etching roughened or oxidized the surface and neither resulted in improved adhesion (Silverstein et al., 2003). Aramids Aramid fibers (Fig. 11.6) have unique properties with very high tensile strength and modulus. Kevlar is the most well-known of all the aramids. Its outstanding potential is derived from the anisotropy of the superimposed substructures presenting pleated, crystalline, fibrillar and skin–core characteristics. Nomex, Twaron and Technora are the other commercially available aramid fibers. The regularly positioned amid segments in Kevlar result in strong hydrogen bonds facilitating good load transfer between chains. These hydrogen-bonded chains form sheets and are stacked parallel into crystallites and van der Waals forces act between these planes (Rebouillat, 2001). Owing to the difference in orientation and alignment of the skin chains versus the core microfibrils, that are substructured by crystallites, Kevlar fibers show a skin–core structure. Aramid fibers are very suitable as reinforcements in high-performance composite materials because they combine a high specific modulus and high strength with a high thermal resistance, high chemical inertia and low electrical conductivity (de Lange et al., 2001). However, the surface of aramid fiber is chemically inert and smooth. Therefore various approaches to the surface modification of aramid fibers have been developed, such as chemical and physical

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treatments. Aramid fibers do not provide satisfactory adhesion to rubber when coated with a conventional RFL system. As in polyamides, the development of adhesion by hydrogen bonding is difficult as amide groups are hidden between aromatic groups. The two-dip system developed by Dupont is the most common adhesive system. The system involves treating a twisted cord with an epoxy resin, followed by a second coating of RFL emulsion. This process provides a strong bond between the aramid surface and the epoxy layer and the rubber matrix (Chawala, 1994). Zhang et al. (2007) studied the surface modification of aramid fibers with γ-ray radiation to improve the bond strength with epoxy resin. They treated Armos aramid fiber with Co60 γ-ray radiation. Two kinds of gas media, N2 and air, were chosen; after treatment the ILLS values of aramid–epoxy composites were enhanced by about 17.7% and 15.8% respectively. XPS analysis showed an increase in the ratio of oxygen/carbon, and AFM and SEM analysis indicated increased surface roughness. Rare earth modifiers and the epoxy chloropropane (ECP) grafting modification method were also used in the rare earth solution (RES) surface treatment of F-12 aramid fiber (Wu and Cheng, 2006). RES surface treatment showed almost no effect on the tensile strength of single fiber and resulted in better interfacial adhesion between aramid fiber and epoxy matrix owing to reactive functional groups on the fiber surface. –COOH groups were introduced on the surface of the aramid fibers by RES treatment. The rare earth compounds are adsorbed onto the aramid fiber surface through chemical bonding increasing the concentration of reactive functional groups on the fiber surface. Twaron® aramid fibers are adhesion activated by the application of an epoxy-based finish to their surface. The resulting fiber surface has improved interfacial adhesion to epoxy and aramid matrices (Willemsen et al., 1987). Allred et al. (1983) introduced amine groups onto the aramid fiber surface by exposure to ammonia plasmas. Amine groups with epoxides formed strong covalent bonds at the composite interface. Yue and Padmanabhan (1999) treated Kevlar fiber surfaces with acetic anhydride and then immediately washed with distilled water. In a second treatment, after acetic anhydride application methanol washing was carried out. The second treatment gave the highest ILSS value. This improvement in the ILSS value was explained by the presence of an oxygen-rich fiber surface. Wertheimer and Schreiber (1981) used microwave plasma in atmospheres of O2, N2 and Ar to oxidize the surface of Kevlar. They observed an increase in the strength of the composite laminates. However, the mechanical properties of single Kevlar fibers deteriorated after plasma exposure; on the other hand, the cohesive strength of the multiflament cloth was increased. A combined plasma and coupling agent treatment with silicone adhesive was also found to be effective in promoting adhesive bonding between aramid fibers and silicone rubber (Inagaki et al., 1992). The plasma treatment resulted in

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elimination of the carbonized layer from the poly(phenylene terephthalamide (PPTA) yarn surface. Oxygen funtionalities such as C–O and C=O groups formed. Tarantili and Andreopoulus (1997) studied the surface treatment of aramid fibers by immersion in a solution of methacryloyl chloride in carbon tetrachloride. After treatment of Twaron 1000® fiber with methacryloyl chloride, small flaws and randomly distributed grooves were created on the fiber surface. These surface changes enhanced the mechanical interlocking with the epoxy matrix resulting in better adhesion. Lin (2002) used metalation, bromination and grafting to modify the surface of Kevlar fiber. The results showed that the ILSS of the bromoacetic acid-grafted Kevlar sample increased by 12% while that of the epichlorohydrin-grafted Kevlar fiber increased by 8%. Bromine-etched Kevlar fiber showed a rougher surface than the untreated fiber. Benrashid and Tesoro (1990) demonstrated the feasibility of introducing functional groups on Kevlar surfaces by electrophilic substitution reactions (nitration and chlorosulfonation). Functional groups were introduced on the surface of fibers modified by nitration and subsequent reduction of nitro groups to amino groups. Chlorosulfonil groups were introduced on the surface after surfacecontrolled chlorosulfonation reactions. All reactions added functional groups on the fiber surface and improved adhesion. Oxygen plasma treatment of high-performance rigid-rod polymeric fibers – PBO, Kevlar and carbon – was carried out and surface free energy components were evaluated by a dynamic contact angle system (Wu, 2004). The results showed that oxygen plasma treatment was effective for surface modification. The total surface energy was increased by 33% for Kevlar fibers, 47% for carbon fibers and 41% for the PBO fiber. The surface O/C ratios were increased for all the fibers.

11.3.4 Carbon fiber Carbon fibers have been continuously developing over the last few decades. Because of the rich variety of carbon fibers available today, their properties also vary. The basic structure of a typical carbon fiber (polyacryonitrile(PAN)) consists of long primary units lying parallel to the fiber axis and bonding together to form a stretched network of branched fibrils that run the full length of the fiber. During the fiber formation process, carbon elements in the outside surface have different attraction forces from the internal, well-structured carbon elements. Therefore, the surface reactivity of the carbon fiber depends on the number of unbalanced carbon elements. Surface properties of carbon fibers strongly depend on the precursors and the formation processes used. Fibers based on rayon have an irregular crosssectional shape and many grooves on the surface. PAN-based carbon fiber has a circular or kidney appearance with some small pits on the surface. High-modulus carbon fibers, however, have a smooth surface. The surface chemistry of the

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carbon fiber is controlled by the oxygen-containing functional groups (Tang and Kardos, 1997). The unique combination of chemical and physical properties of carbon fibers has led to their introduction in many different application areas. Carbon fibers are now considered as the main reinforcement material for high-performance polymer matrix composites. However, the smooth and inert characteristics of the carbon fiber surface result in weak adhesion to the polymer matrix. Several methods have been developed for the surface modification of carbon fibers. Xu et al. (2008) modified carbon fibers by the free-radical polymerization method and acrylic acid (AA) was grafted onto the carbon fiber surface. The initiator was produced by the redox-induced reaction of KMnO4/H2SO4 It was confirmed that grafting AA led to a remarkable increase in oxygen-containing functional groups on the fiber surface. The ILSS value of the composite was enhanced by 17.3%. The original carbon fiber surface has a relatively smooth surface with narrow grooves parallel to fiber axis. After grafting many tiny fragments of acrylic copolymer stick to the fiber surface. The authors suggested that this improved the absorbability of the treated fibers, providing more effective wetting of the fiber and matrix. Bismarck et al. (1999) studied the grafting of methacrylic acid onto carbon fiber to improve adhesion to a polymer matrix. Unmodified carbon fibers and oxygenplasma-treated fibers were grafted with methacrylic acid. Grafting of a water-soluble polymer onto carbon fibers worsened the wettability with water; however the wettability towards the non-polar liquid diiodomethane was improved. The surface polarity decreased for all systems studied. Tsubokawa et al. (2002) reported that the grafting reaction of poly(vinyl ferrocene-co-methyl methacrylate) (poly-VFE-co-MMA) onto a carbon fiber surface was achieved by a ligand-exchange reaction between the ferrocene moieties of the copolymer and polycondensed aromatic rings of the carbon fiber. The hydrophobic nature of the carbon fiber surface is increased by grafting of the hydrophobic copolymers onto the surface. Akovali and Dilsiz (1996) studied the surface of carbon fibers following different plasma treatments of xylene–air, dioxane, allylcyanide, and toluene–air– argon. The surface-treated carbon fibers yielded composites with better ILSS and flexural strength. Lu et al. (2007) studied the surface modification of carbon fiber using air plasma application. The results indicated that air plasma treatment was capable of increasing surface roughness as well as introducing surface polar groups onto the carbon fiber. They found mechanical interaction to be more effective on composite interfacial adhesion than polar groups on the surface and this was responsible for the increase in ILSS of carbon fiber–poly (phthalazinone ether sulfone ketone) (PPESK). When compared with other systems such as surface heat treatment, sizing, HNO3 oxidation, electrochemical oxidation, gas-phase oxidation, ultrasonic bombardment irradiation treatment, etc., plasma treatment was an effective method of increasing fiber polarity as well as surface roughness.

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Surface modification of high-modulus carbon fiber was studied by Severini et al. (2002). The reactions that they used for surface modification are Diels-Alder, additions of maleic acid anhdride (MA) or tetracyanoethylene (TCE), and nucleophilic substitution of hydroxyl groups with ammonia. After TCE and MA treatments the oxygen and nitrogen atoms in the fiber increased. Amino groups were introduced by ammonia treatment. Rearick and Harrison (1995) modified a pitch-based carbon fiber surface using a nickel-catalyzed oxidation treatment. They showed an increase in the IFSS of the fiber–matrix system. The surface physico-chemical structure of carbon fibers – for example, chemical composition, microstructure, morphology, surface area, cross-sectional shape, surface free energy – is dependent on the precursors (PAN, rayon and pitch). Carbon fibers have a high quantity of active sides on the surface and the surface has low crystallite orientation; these lead to easy surface modification. In contrast, the high-modulus carbon fibers have low reactivity and their modification is relatively difficult (Tang and Kardos, 1997). Jin et al. (1994) introduced functional groups onto PAN-based carbon fibers by oxygen plasma treatment. The amount of functional groups on the fiber surface initially increased with increasing treatment time up to 2 min; however, after this time the amount remained constant. The plasma treatment increased the surface area but mechanical performance decreased with increasing treatment times.

11.4

Surface modification of textiles for filtration

11.4.1 Principles of filtration Filtration is a separation process where one material is separated from another. Textile materials are widely used in the filtration of air and liquids. Textile fibers are mainly applied to solid–gas and solid–liquid separation. Three elements are involved in any filtration system: the particles, the fluid medium and the filter. The filter system can be characterized with respect to the particle (size, shape, mass change), the fluid medium (velocity, viscosity, density, temperature) and the filter (cross-sectional shape, linear density, type, orientation, charge) (Rubenstein and Koehl, 1977). Textile materials, especially woven and nonwoven fabrics, are suitable for filtration. They are a three-dimensional network of fibers, have considerable thickness and give high filtration efficiencies (Adanur, 1995). The efficiency of a filter is directly related to the particle size of the material to be filtered. The larger the particle size for a particular filter, the higher the efficiency. Permeability is the capacity of a porous medium to transmit fluids. If the dimensions of the particle to be filtered are larger than the pore size, the particle is stopped easily. For particles that are smaller than the size of the pores, there are five mechanisms by which these particles can be caught (Adanur, 1995; Rubenstein and Koehl, 1977).

258 1 2

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Surface modification of textiles Direct interception: when a particle tries to pass the fiber surface at a distance smaller than the particle radius, it will contact the fiber and be captured. Inertial impaction: particles with mass tend to follow streamlines until the fluid is diverted and accelerated as it passes around the fiber because of the continuity equation; then due to their own inertia, the particles are thrown out of the flow streamlines and can be caught by other fibers. Diffusion (Brownian motion): very small particles show random motion as they collide with molecules of the surrounding medium; their zigzag route increases the chance of being caught by the fibers. Electrostatic attraction: if the fiber and the particle are of opposite electrical charge, the particle can be attracted to the fiber and captured. Increasing the attractive force between the fiber and the particle by increasing the charge of either will increase the filtration efficiency. Gravitational deposition: particles denser than the fluid will sink and may collide with the fibers.

The main purpose of filtering media is to capture particles of different sizes. These five mechanisms of filtering may work at the same time or one of them may dominate depending on the particle character (e.g. size and the type of electrical charge). As particle size is increased, inertial impaction, gravitational deposition and direct interception are improved; on the other hand as particle size is decreased, collection by diffusion is enhanced. Lee and Liu (1982) showed that in the sub-micrometer particle size range, diffusion is the dominant collection mechanism. For any given particle size, when velocity is decreased gravitational deposition is improved, as velocity increases inertial impaction of particles is improved. Depending on the separation process and the particle size, filtration can be achieved by particle filtration, microfiltration, ultrafiltration, nanofiltration and hyperfiltration techniques. Steffens and Coury (2007) studied the particle collection efficiency of a polyester fiber filter. They also showed that particle efficiency decreases with increasing particle diameter and with increasing fluid velocity. Filtration can be also classified according to the filtration environment, i.e. wet and dry filtration. Depending on the application, different-shaped industrial filters are used on drums, discs, plates, frames, belts and vessels. The efficiency of particle capture depends on various parameters. Textile structures give best filtration efficiencies due to their three-dimensional structure. Their open structure also allows the flow of fluid (gas or liquid) without a reduction in the flow pressure. When designing a textile structure for filtration, several factors should be considered, for example flow velocity, pressure inside the filtering system, and size and concentration of the particles. The fibers used in filters should be able to withstand the high temperatures, chemical conditions, mechanical deformations, etc. that they will be exposed to in use. The three-dimensional isotropic nature of the textile structure increases the

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Filtering media Membranes

Textile structures Woven fabrics

Knitted fabrics Nonwovens Conventional fibers

Nano fibers

Spun-bonded

Melt-blown

11.7 Filtering media hierarchy.

probability of particles being caught by the fibers. Therefore, nonwoven textile structures provide higher filtration efficiencies than woven or knit fabrics, which are two-dimensional structures. Synthetic fibers are more commonly used for filtration than natural fibers as they can withstand harsher filtering conditions (Fig. 11.7). Sanchez et al. (2007) also studied the effect of fiber diameter on filtration efficiency. They compared nonwoven fabric filters made of two different fiber cross-section shapes. The experimental results showed that the particle collection efficiency of a filter made of polyimide fibers with a trilobal shape was higher than that of polyester fiber filters with circular cross-section. Steffens and Coury (2007) studied the collection efficiency of polyester fiber filters for nano-sized aerosol particles with sizes varying from 8.5 to 94.8 nm. The results showed that the efficiency decreased with increasing particle size, indicating the predominance of the diffusional mechanism. The filter efficiency also decreased with filtration velocity. Lamb et al. (1975) studied the effects of five fiber properties on the filtration performance. The variables studied were cross-sectional shape, linear density, surface roughness, crimp and fiber length. They showed that use of trilobal rather than round fibers improved filtration efficiency, crimped fibers also improved the efficiency. Fibers with rougher surfaces were more efficient in removing finer particles. The efficiency was also improved by use of 3-denier rather than 6-denier fibers but at the cost of greater drag. Vaughn and Carman (2001) investigated the effect of cross-sectional shape on filtration efficiency. They used 4 DG, a deep-grooved surface polyester fiber with eight channels. The grooved surface of the fiber enables liquid transport along the length of individual fibers and also has a higher surface area per unit weight; it gives bulkier textile structures. These fibers also provided more particle capture resulting in higher filtration efficiency.

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Filtration efficiency depends on selecting the proper filtering media (Fig. 11.7). Membranes and textile structures are used as filtering media. Nonwovens are widely used within textile structures for the reasons discussed earlier. By introducing electrospinning of nano-sized fibers, nonwoven filtration surfaces are increasingly used for filtration as they provide greater surface area per weight. This enables more particles to be captured by the fiber surface. Permeability, filtration efficiency, filtration cost and filtration life are the important parameters to consider in choosing the right structure for optimum filtration. Mayer and Warren (1998) compared polymeric membranes and nonwovens. Membranes are produced by a wet-cast process or biaxial stretching. Nonwovens, however, are manufactured by a melt-spinning process. Solution spinning is used more for electrospinning of nanofibers. Polymer is extruded through spinneret holes on a surface. Microporous membranes and spun-bonded filters have decreased permeability while melt-blown nonwovens and nanofiber filters give a high level of permeability. Membranes provide the most efficient filtration and are the most expensive. Melt-blown filters on the other hand are the cheapest.

11.5

Applications of surface-modified fibers used for filtration

The most widely used fibers in filtration are polyester fibers. They show relatively good strength, high temperature resistance and low cost. However, polyester fibers have low resistance to alkalis, acids and steam; hot water and steam hydrolyze the fiber due to ester linkages. On the other hand, polyester fibers with different linear densities and cross-sectional shapes can be produced easily. Nomex®, Teflon and glass fibers are used in high-temperature filtration; glass fibers also have a high particle holding capacity; ceramic fibers are suitable for hot glass filtration even though they have a high cost. Polypropylene is the second most widely used fiber for filtration. It is a hydrophobic fiber and has excellent acid, alkali and abrasion resistance. It is very suitable for melt-blown and spun-bond nonwovens. However it has a relatively low melting temperature. Agranovski and Braddock (1998) studied filtration of liquid aerosols on nonwettable fibrous filters. They used Teflon and polypropylene fibers, both of which are hydrophobic and have non-wettable surfaces. These non-wettable fibers forced the liquid particles to form droplets on the fibers; these droplets could then be drained and collected. The authors showed that these droplets blocked the flow of glass. They then reached a critical size at which they began to oscillate, then broke free and fell from the filter. This self-cleaning nature of the non-wettable filter has important applications in the filtering of liquid aerosols. The wettability/nonwettability of the filter fibers in relation to the liquid aerosol is one of the most important parameters of liquid aerosol filtration. It has been shown that liquid aerosols will adhere to wettable filter fibers and form liquid films around the fibers.

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Needled nonwoven filter media are also used for filtration. Singeing, calendering and microporous coatings are used to modify the filter fabric surface. Singeing and calendering are the first modification techniques. Singeing is suitable for fabrics made from short stable fibers. The fabric is quickly passed over a gas flame or brought into rapid contact with a metal strip heated to a very high temperature. Short protruding fibers are then overcome, resulting in a smoother surface; this provides easy cake discharge. The calendering process improves the surface smoothness of the fabric and regulates the fabric’s permeability. Needled nonwovens are coated with a polymer emulsion to create a thin microporous coating; this improves cake release and filtration properties (Lydon, 2004). Lawrence and Liu (2006) extensively studied the relationship between the structure, permeability and filtration performance of needled nonwoven filter media. They studied 18 different nonwoven fabrics. The fiber types were polypropylene, polyester (PET), aramid and Nylon (PA). Four different surface treatments were applied to the surface of the fabrics. Microporous coated, laminated e-polytetrafluoroethylene (PTFE), heated calendered and singed materials were compared. The coated and laminated materials had low permeability values. Fiber-compressive modulus and the effect of temperature on the fiber were very important for the calendered surface finish as pressure was applied to the surface under high temperatures. Polypropylene fibers had the lowest modulus and the glass transition temperature showed more flattening. Goosens (1993) studied the improvement in the surface structure of filter media when using bonding, laminating, coating and impregnating techniques. Powder was scattered onto the surface of the nonwoven by a calender equipped with a powder applicator. During calendering the powder became molten and formed a layer. Goosens applied metastable foam to nonwovens by means of a rotary screen; some of the air bubbles burst and a controlled open structure resulted. This improved the surface of cleanable gas filters. Martel et al. (2002) grafted cyclodextrins (CD) onto polypropylene nonwoven fabrics for the manufacture of reactive filters. Textile materials functionalized with CDs demonstrate improved capture of active or polluting substances. Interactions between the organic substrates and the filter through specific interactions with CDs and different mechanisms of sorption occur. The first fraction of the substrate was trapped by CD cavities, also hydrogen bonding between the solute and CDs and hydrophobic guest–guest interactions occurred. Glass fiber used in filtration applications is produced by one of three types of processes: (a) continuous draw; (b) centrifugal spinning; or (c) flame attenuation. Each process gives different characteristics to the fiber, for example diameter, length, surface chemistry and structure. For diameter and length, it is the distribution of these values that is the most relevant measure. Diameter distribution affects the filtration efficiency, whereas fiber length distribution affects the tensile and tear strength. Continuous draw spinning produces a controlled fiber diameter, while centrifugal spinning produces fibers with

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diameters of about 2–8 µm. Flame attenuation spinning produces the finest glass filtration. There is quite a difference in ‘mean diameters’; continuous draw spinning gives smaller surface differences between fibers as the finer diameter fibers cool faster. Flame attenuation fibers have the highest surface area. Bauer and Manville (2004) concluded that by controlling initial glass chemistry and the fiberization process parameters, a wide range of fiber surface properties can be achieved. This enables the glass fibers to be applied in many different areas. Corona discharge was applied on melt-blown nonwoven polypropylene (Yang et al., 2002). The samples were charged under a high electric field for 2–10 minutes, and then put in an air conditioner to evaluate their filtration efficiency for indoor air. The charged nonwoven fabric showed better filtration compared with the uncharged nonwoven fabric. The charged nonwoven polypropylene collected more particles.

11.6

Future trends

Fiber-reinforced composites are used in many different areas as they provide very strong but lightweight materials. The composites can now be found in sporting goods, automobile bodies, aerospace parts, defense and military applications, civil and structural applications (such as bridges) and complete composite buildings. Most commercial fiber-reinforced composites are made from petroleum-based synthetic fibers and resins. It has been estimated that at current consumption rates, worldwide petroleum reserves will last for the next 50 years or less (Stevens, 2002). As the supplies decline, prices are expected to increase to a great extent. Petroleum-based composites are also not biodegradable; environmentally friendly, fully biodegradable ‘green’ composites, based on plant fibers and resins are increasingly being developed for various applications (Chabba et al., 2005). The new generation of biobased polymeric products is based on renewable biobased plant and agricultural stock and forms the basis for sustainable, eco-friendly products. Combinations of natural fibers (like kenaf, hemp, flax, jute, cotton, sisal and pineapple leaf fiber) with polymer matrices from renewable resources to produce green composites has been gaining attention in recent years. Currently some natural fibers such as hemp, kenaf and sisal are being utilized commercially in biocomposites with polypropylene for automotive applications. Natural fibers are now emerging as feasible alternatives to man-made fibers in composites. The advantages of natural fibers include recyclability, biodegradability and low cost. However, they also have some disadvantages. The hydrophilic nature of natural fibers adversely affects adhesion to a hydrophobic matrix resulting in poor strength. They have no regular cross-section and have many defects caused by twisting in the stacking of cellulosic chains. The stacking of cellulosic chains causes knees at the fiber surface and creates weak points. The cellulosic fibers also swell when treated with polar media such as water. However, a hydrophobic matrix does not change its dimensions when treated with water and

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therefore the swelling of the cellulosic fibers results in uneven stress distribution. The highly polar nature of natural fibers also makes them incompatible with nonpolar matrix polymers. Since natural fibers have many advantages over synthetic fibers, investigations should be carried out into the manufacturing of a reactive surface for natural fibers through low-cost but effective surface treatments without changing the bulk properties. Alkali treatment is an effective method for improving fiber–matrix adhesion in natural fibers due to the fibrillation effect. Another trend in the field of composites is the reduction of cost. This cost reduction not only relates to the improvement and optimization of the processes but also to the need for integration between design, material, process, tooling and manufacturing. Significant breakthroughs are expected in new composite materials in areas where functionality is the most relevant technical requirement. The introduction of smart materials, advanced materials, active materials and nanomaterials will enable us to produce functional composites. Human quality of life, for example, would be greatly improved by the availability of artificial prostheses and organs; these would restore, repair or replace the structural and functional properties of the natural tissues. Nanotechnology is expected to lead to high-strength, lightweight polymer nanocomposites because of the properties evident in nano-scale reinforcing elements. Conventional fiber-reinforced composites are being replaced by nanofiber-reinforced composites as they offer larger adhesion surfaces. However, due to the absence of homogeneous diameter distribution along the fiber length and differences in fineness between the fibers, the strength of these composites is limited. Therefore more research on the production of nanofibers for composite applications should be carried out. Surface properties of reinforcing fibers used in composites are very important as they affect the adhesion between the matrix and the fiber. Most of the fibers require surface modification owing to their low surface energy and hydrophobic nature. Cheaper and eco-friendly surface modification methods have received great attention from researchers. People are becoming increasingly aware of the environment and environmental issues. They require tougher legislation to protect the environment and provide a healthier and more comfortable indoor climate in public areas, commercial buildings, manufacturing facilities, hospitals and cars. Much higher filtration efficiencies are therefore required and since the filters must meet increasingly tough performance requirements, a serious need has emerged to find new materials and new manufacturing and testing methods. Concerns about the environmental impact of manufacturing, in terms of both energy use and reducing cost, have led to new guidelines for cost and environmental analysis of filters (Gustavsson, 2006). Nanofibers have some extraordinary properties that mean they are predestined for highly effective air filtration. Nanofiber materials have a high specific surface, low weight, small fiber diameter and high porosity. The range of pore sizes ensures

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that microparticles, bacteria or even viruses cannot pass through the nanofiber material. Low weight leads to economic production, thanks to material savings, and the small diameters of the fibers ensure high filtration effectiveness, while obtaining a low pressure drop.

11.7

References

ADANUR S (1995), Wellington Sears Handbook Of Industrial Textiles, Technomic Publishing Company, Inc., Pennsylvania. AGRANOVSKI I E AND BRADDOCK R D (1998), ‘Filtration of liquid aerosols on nonwettable fibrous filters’, AIChE Journal, 44(12), 2784–2790. AKOVALI G AND DILSIZ N (1996), ‘Studies on the modification of interphase/interfaces by use of plasma in certain polymer composite systems’, Polymer Engineering and Science, 36(8), 1081–1086. ALLRED R E, MERRILL E W AND ROYLANCE D K (1983), In Ishida H and Kumar G (Eds), Molecular Characterization of Composite Interfaces, Plenum Press, New York, pp. 333– 375. AYTAC A, SEN M, DENIZ V AND GUVEN O (2007), ‘Effect of gamma irradiation on the properties of tyre cords’, Nuclear Instruments and Methods in Physics Research B, 265, 271–275. BAUER J F AND MANVILLE J (2004), ‘Properties of glass fiber for filtration: Influence of forming process’, INJ, Winter, 2–7. BENRASHID R AND TESORO G C (1990), ‘Effect of surface-limited reactions on the properties of Kevlar fibers’, Textile Research Journal, 60, 334–344. BISMARCK A, PFAFFERNOSCHKE M AND SPRINGER J (1999), ‘Grafted carbon fibers and their physicochemical properties. I. Grafting of methaacrylic acid’, Journal of Applied Polymer Science, 71, 1175–1185. CARLOTTI S AND MAS A (1998), ‘Improvement of adhesion of PET fibers to rubber by argonoxygen plasma treatment’, Journal of Applied Polymer Science, 69, 2321–23230. CHABBA S, MATTHEWS G F AND NETRAVALI A N (2005), ‘Green composites using crosslinked soy flour and flax yarns’, Green Chemistry, 7, 576–581. CHAN C (1994), Polymer Surface Modification and Characterization, Hanser/Gardner Publications, Cincinnati. CHAPPELL P J C, BROWN J R, GEORGE G A AND WILLIS H A (1991), ‘Surface modification of extended chain polyethylene fibers to improve adhesion to epoxy and unsaturated polyester resins’, Surface and Interface Analysis, 17(3), 143–150. CHAWALA S K (1994), Rubber composites, in Brody H (Ed.), Synthetic Fibre Materials, Longman Scientific and Technical, Essex, England. CHOI H S, SHIKOVA T G, TITOV V A AND RYBKIN V V (2006), ‘Surface oxidation of polyethylene using an atmospheric pressure glow discharge with liquid electrolyte cathode’, Colloid and Interface Science, 300, 640–647. CIOFFI M O H, VOORWALD H J C AND MOTA R P (2003), ‘Surface energy increase of oxygenplasma-treated PET’, Materials Characterization, 50, 209–215. DE LANGE P J, AKKER P G, MAAS A H J, KNOESTER A AND BRONGERSMA H H (2001), ‘Adhesion activation of Twaron aramid fibres studied with low-energy ion scattering and x-ray photoelectron spectroscopy’, Surface and Interface Analysis, 31, 1079–1084. GARBASSI F, MORRA M AND OCCHIELLO E (1994), Polymer Surfaces From Physics to Technology, John Wiley & Sons Ltd, West Sussex.

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GOOSENS F (1993), ‘Improvement of surface structure of filter media by bonding, laminating, coating and impregnating’, Journal of Coated Fabrics, 22, 279–289. GUSTAVSSON J (2006), Trends in air filtration, ASHRAE Journal 2005–2006, ASHRAE. HOCKENBERGER A S AND KORAL S (2007), Research Project, 106M131 TUBITAK, Turkey. HOLMES S AND SCHWARTZ P (1990), ‘Amination of ultra-high strength polythylene using ammonia plasma’, Composites Science and Technology, 38(1), 1–21. HSIEH Y (2001), Surface characteristics of polyester fibers, in Pastore C M and Kiekens P (Eds), Surface Characteristics of Fibers and Textiles, Marcel Dekker, New York-Basel, pp. 34–55. INAGAKI N, TASAKA S AND KAWAI H (1992), ‘Surface modification of Kevlar® fiber by a combination of plasma treatment and coupling agent treatment for silicone rubber composite’, Journal of Adhesion Science and Technology, 6(2), 279–291. IVAN H, MICHAL J, HENRICH K, MIRKO C AND VIERA S (2006), ‘Plasma treatment and polymerization of textile reinforcing materials’, e-Journal of Soft Materials, 2, 23–30. JAMSHIDI M, AFSHAR F, MOHAOMMADI N AND POURMAHDIAN S (2005), ‘Study on cord/ rubber interface at elevated temperatures by H-pull test method’, Applied Surface Science, 249, 208–215. JENNINGS C W (1972), ‘Surface roughness and bond strength of adhesives’, Journal of Adhesion, 4, 25–40. JIN B S, LEE K H AND CHOE C R (2003), ‘Properties of carbon fibers modified by oxygen plasma’, Polymer International, 34(2), 181–185. KOVAC F J AND RODGERS M B (1994), Tire Engineering, in Mark J E, Erman B and Eirich R F (Eds), Science and Technology of Rubber, Academic Press, New York, pp. 675–715. KRUMP H, HUDEC I, JASSO M, DAYSS E AND LUYT A S (2006), ‘Physical-morphological and chemical changes leading to an increase in adhesion between plasma treated polyester fibres and a rubber matrix’, Applied Surface Science, 252, 4264–4278. LADIZESKY N H AND WARD I M (1983), ‘A study of the adhesion of drawn polyethylene fiber polymeric resin systems’, Journal of Materials Science, 18(2), 533–544. LAMB G E R, COSTANZA P AND MILLER BERNARD (1975), ‘Influences of fiber geometry on the performance of nonwoven air filters’, Textile Research Journal, 45, 452–463. LAWRENCE C A AND LIU P (2006), ‘Relation of structure, properties and performance of fibrous media for gas filtration’, Chemical and Engineering Technology, 29(8), 957–967. LEE K W AND LIU B Y H (1982), ‘Theoretical study of aerosol filtration by fibrous filters’, Aerosol Science and Technology, 1(2), 147–166. LEHOCKY M, DRNOVSKA H, LAPCIKOVA B, BARROSTIMMONS A M, TRINDADE T, ZEMBALA M AND LAPCIK L (2003), ‘Plasma surface modification of polyethylene’, Colloid and Surfaces A: Physicochem. Eng. Aspects, 222, 125–131. LI Z F AND NETRAVALI A N (2003), ‘Surface modification of UHSPE fibers through allylamine plasma deposition. II Effect on fiber and fiber/epoxy interface’, Journal of Applied Polymer Science, 44(2), 333–346. LIN J (2002), ‘Effect of surface modification by bromination and metalation on Kevlar fibreepoxy adhesion’, European Polymer Journal, 38, 79–86. LU C, CHEN P, YU Q, DING Z, LIN Z AND LI W (2007), ‘Interfacial adhesion of plasma-treated carbon fiber/poly(phthalazione ether sulfone ketone) composite’, Journal of Applied Polymer Science, 106, 1733–1741.

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LYDON R (2004), ‘Filter media surface modification technology: state of the art’, Filtration and Separation, November, 20–21. MARTEL B, THUAUT P, BERTINI S, CRINI G, BACQUET M, TORRI G AND MORCELLET M (2002), ‘Grafting of cyclodextrins onto polypropylene nonwoven fabrics for the manufacture of reactive filters’, Journal of Applied Polymer Science, 85, 1771–1778. MAYER E AND WARREN J (1998), ‘Evaluating filtration media: A comparison of polymeric membranes and nonwovens’, Filtration and Separation, December, 912–914. MIRACLE D B AND DONALDSON S L (2006), Introduction to Composites, Air Force Research Laboratory, USA. MOON S AND JANG J (1997), ‘A study of the impact properties of surface-modified UHMPEfiber/vinylester composites’, Composites Science and Technology, 57, 197–203. NETRAVALI A N, CACERES J M, THOMPSON M O AND RENK T J (1999), ‘Surface modification of ultra-high strength polyethylene fibers for enhanced adhesion to epoxy resins using intense pulsed high-power ion beam’, Journal of Adhesion Science and Technology, 13(11), 1331–1342. OKAMURA Y, TAGAWA M, GOTON K, SUNAGA M AND TAGAWA T (1996), ‘Influence of drawing and heat treatment on surface free energies of polyethylene terephthalate fibers’, Colloid and Polymer Science, 274(7), 628–633. PACKHAM D E (2003), ‘Surface energy, surface topography and adhesion’, International Journal of Adhesion and Adhesives, 23, 437–448. REARICK B K AND HARRISON I R (1995), ‘Modification of pitch-based carbon fibers using a nickel-catalyzed oxidation treatment: Effect of treatment on fiber-matrix interfacial shear strength’, Polymer Composites, 16(2), 180–188. REBOUILLAT S (2001), Aramids, in Hearle J W S (Ed.), High Performance Fibres, Woodhead Publishing Ltd, Cambridge, UK, pp. 23–61. RHEE K Y, CHOI J H AND PARK S J (2005), ‘Effect of 1 keV Ar+ irradiation on the residual strength of PE fiber-reinforced composites’, Materials Science and Engineering A, 395, 288–294. RUBENSTEIN D I AND KOEHL M A R (1977), ‘The mechanisms of filter feeding: Some theoretical considerations’, American Naturalist, 111, 981–994. SANCHEZ J R, RODTIQUEZ J M, ALVARO A AND ESTEVEZ A M (2007), ‘The capture of fly ash particles using circular and noncircular cross-section fabric filters’, Environmental Progress, 26(1), 50–58. SEVERINI F, FORMARO L, PEGORARO M AND POSCA L (2002), ‘Chemical modification of carbon fiber surfaces’, Carbon, 40, 735–741. SILVERSTEIN M S, BREUER O AND DODIUK H (2003), ‘Surface modification of UHMWPE fibers’, Journal of Applied Polymer Science, 52(12), 1785–1795. SONG Q AND NETRAVALI A N (1998), ‘Excimer laser surface modification of ultra-highstrength polyethylene fibers for enhanced adhesion with epoxy resins. Part 2. Effect of treatment environment’, Journal of Adhesion Science and Technology, 12(9), 983–998. STEFFENS J AND COURY J R (2007), ‘Collection efficiency of fiber filters operating on the removal of nano-sized aerosol particles: I-Homogeneous fibers’, Seperation and Purification Technology, 58, 99–105. STEVENS E S (2002), Green Plastics, Princeton University Press, Princeton, USA. TABOUDOUCHT A, OPALKO R AND ISHIDA H (2004), ‘Effect of fuming nitric acid surface treatment of ultra-high modulus polyethylene fibers on the mechanical properties of their composites’. Polymer Composites, 13, 81–85. TANG L AND KARDOS J L (1997), ‘A review of methods for improving the interfacial

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12 Surface modification of textiles by aqueous solutions J. WANG

The Procter & Gamble Company, USA J. LIU

Zhejiang Sci-Tech University, China Abstract: The majority of textile surface modifications are achieved using aqueous solutions. Removal of impurities and additives from a fabric surface often happens in acidic or alkali solutions. Oxidation or reduction can accelerate the removal process and can often introduce new chemical groups on the fiber surface. Functional compounds or polymers can be added to the fabric surface to provide different functions which could be durable or nondurable to home laundering. Six important examples are discussed in this chapter: cotton mercerization, moisture management, stain repellency, fabric softener, polyester surface hydrolysis and wool shrinkage control. Key words: textile surface modification, textile wet process, fiber hydrolysis, fiber oxidation, textile functional finish.

12.1

Introduction

The majority of textile surface modifications are achieved using aqueous solutions. The surface modifications are provided either by physical interactions (which do not involve chemical reactions with fibers) or by chemical interactions (which involve chemical reactions). The physical interactions include the removal of fiber impurities and materials added during fiber and fabric processes. The physical interactions also include depositions of any materials that may change fabric properties. Adding fabric softeners at the end of the textile process is a common practice to change fabric smoothness and fabric hand. Adding hydrophilic agents has become a useful approach to improve fabric moisture management and soil-release properties, particularly for synthetic fibers. A stainrepellent finish is a good example of changing the fabric surface energy to improve the fabric’s repellency to common consumer stains including wine, coffee and other beverages. The stain-repellent finish often improves fabric soil-release performance, too. The chemical interactions involve chemical reactions on the fiber surface. Bleaching is a traditional textile process to whiten a fabric surface and to get textiles ready for dyeing and finishing. It not only changes the fabric appearance, 269

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but also fabric absorbance and even fabric mechanical properties. Bleaching can be an oxidation process or a reduction process, depending on the fiber contents and the textile end-use requirements. Grafting has become a popular approach for adding chemicals to a fabric surface to change various fabric properties. Many compounds and polymers can be grafted onto the fiber surface. Since grafting often creates covalent bonds between compounds and the fiber surface, grafting often therefore provides durable functions or unique performance characteristics. Surface modifications of textiles in aqueous solutions are often fiber specific. Different fibers usually need different materials or chemistries to achieve the most effective fiber surface modification and the best textile performance. Recently, polyester fiber has overtaken cotton fiber as the top textile fiber type in terms of all global textile applications – including apparel, home textiles, industrial textiles and carpets (Textile Pipeline, 2008). Although cotton remains the top apparel fiber worldwide, polyester is expected to overtake cotton as the number one apparel fiber type in the near future. With the increased popularity of polyester fiber in the global textiles and apparel market, many unique surface modifications have been developed for polyester fibers. By nature, polyester fiber is hydrophobic owing to the lack of any hydrophilic groups in its structure. In order to make apparel made from polyester fibers comfortable for consumers, surface modifications are required to make the fabric surface hydrophilic enough for a good moisture management performance. The recent boom in performance wear or active wear, which are mainly made from polyester and its blends, has accelerated the interest in surface modification applications from both industry and research institutions. Almost all textile wet processes in the textile industry involve fiber surface modifications by aqueous solutions. Traditional wet processes – such as desizing, scouring, bleaching, dyeing and finishing – all mainly use aqueous solutions. A pad–dry–cure method is commonly used for continuous textile processes for high productivity. On the other hand, a batch process is often selected for textiles that need uniform and deep treatments with a higher quality. The sol-gel process (Abidi et al., 2007; Lobnik and Gutmaher, 2006; Schmalz et al., 2003) is a relatively new method for textile surface modifications in aqueous solutions. Recent textile innovations such as nanotechnology, biotechnology and new materials are having an increasing impact on textile surface modifications.

12.2

Mechanisms and chemistries of textile surface modifications

The removal of impurities and additives from a fabric surface often occurs in acidic or alkali solutions. Oxidation or reduction can accelerate the removal process and can often introduce new chemical groups on the fiber surface to change the fabric’s

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absorbance and hydrophilicity. Functional compounds or polymers can be added to the fabric surface to provide different functions that may be durable or nondurable to home laundering.

12.2.1 Surface modifications in acidic or alkali solutions Textile fibers, whether they are natural (cotton, silk, wool, linen, etc.) or man-made (polyester, polyamide, acrylic, rayon, polypropylene, etc.) contain natural or added impurities. For example, a typical cotton fiber contains about 5–10% noncellulose impurities such as proteins, waxes and inorganic compounds. For synthetic and regenerated fibers such as polyester and rayon, many oily processing additives may be present on the fiber surface. These impurities make the textile fiber surface too hydrophobic and not uniform enough for dyeing and finishing so they must be removed at the preparation or pretreatment stages. Sizing agents for woven fabrics – mainly starch, starch derivatives and polyvinyl alcohol – also have to be removed. Alkali solutions are commonly used in textile preparations in order to remove the impurities from the fabric surface effectively. Caustic soda (NaOH), soda ash (Na2CO3) and phosphates are common alkalines used to make alkali solutions, often together with surfactants and sequestering or chelating agents, and sometimes with oxidizing or reducing agents. With alkali solutions, the impurities will swell and may even be hydrolyzed so they can be readily removed by wetting, scouring and detergency actions of surfactants. Cellulose fibers can be readily hydrolyzed in acidic aqueous solutions, leading to a reduced degree of polymerization. Cellulose fibers are, however, relatively stable in alkali aqueous solutions. This is one of the reasons that textiles and apparel are commonly laundered under alkali conditions. On the other hand, protein fibers such as wool and silk are more stable in acidic solutions than in alkali solutions. For hydrophilic fibers such as cotton, silk and rayon, the removal of impurities is often the most useful surface modification for improving the end-use performance of the textiles or improving their processability in the textile industry. On the other hand, the removal of impurities alone is not sufficient for hydrophobic fibers such as polyester. Poly(ethylene terephthalate) (PET) is a dominating polyester form in all polyester fibers. Many different PET surface modifications in alkali solutions have been developed and applied in the industry. PET fiber is one of the most hydrophobic fibers by nature. Its moisture regain is less than 0.5% under the standard relative humidity conditions (65%), leading to poor performance with regard to moisture management, anti-static properties and cleaning for hydrophobic stains, etc. Under the right conditions, the hydrophobic ester groups in polyester fibers can be hydrolyzed to form hydrophilic groups, i.e. carboxylic and hydroxyl groups (Holmes and Zeronian, 1995; Montazer and Sadighi, 2006; Needles et al., 1995).

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OH OH

O

HO

O

O

HO

O

OH OH OH O

H3O+ HO

OH H HO O

O O

+

OH

OH OH

OH

O

+

HO

O

+

HO

OH

H

O HO

OH

OH

OH O

H 2O HO

OH OH H

+

HO

O O

HO

OH

Cellulose degradation in acidic solution

O

O C O

OH– H 2O

C OH

Hydrolysis of polyester fibers in alkali aqueous solution

OH

Surface modification of textiles by aqueous solutions

273

These hydrophilic groups, once created, can be further used to impart different functions to the polyester fibers. For example, N-halamine siloxanes have been coated onto the surface of polyester fibers with hydroxyl groups created by hydrolysis with sodium hydroxide to produce antimicrobial polyester fibers (Ren et al., 2008). Carbon fibers can be attached to an alkali-hydrolyzed polyester fiber surface to produce electric conductive polyester products (Kishida et al., 1987). The surface hydrolysis approach can be used with almost all polyester fiber types. The surface alkali hydrolysis of fibers made from poly(trimethylene terephthalate) (PTT) was studied with the aim of increasing porosity in PTT fibers (Kotek et al., 2004). Recently many studies have been devoted to ‘corn fiber’; this is poly(L-lactic acid) (PLA), a polyester fiber originated from corn. A fabric made from PLA fibers was treated with aqueous sodium hydroxide to give a fabric demonstrating good feel, strength and water/sweat absorption (Isoda and Yasuda, 2002). The changes in the surface morphology of PLA fibers as a result of alkali hydrolysis have been studied by scanning electron microscopy (SEM) and X-ray techniques (Hyon, 1998). One study showed that the PLA fiber hydrolysis could accelerate the apatite formation but had little effect on the chemical and crystalline structure of the apatite (Yuan et al., 2003). Since home laundering mostly occurs in aqueous alkali solutions, polyester fabrics are subject to slow surface hydrolysis which renders the fiber surface more hydrophilic and less prone to the retention and redeposition of oil and fatty stains (Bishop, 1995). The impact is typically noticeable after multiple laundry cycles owing to the slower action of domestic machines compared with those used in the textile industry. Although, not as popular as alkali solutions, aqueous acidic solutions can also be used for fiber surface modifications. One example is using stone washing for cotton fabrics in acidic solutions, particularly for denim fabrics and garments, to introduce a clean, smooth surface appearance and soft hand. The acidic solution can degrade cellulose, leading to a moderate loss of fabric weight and strength. Polyester fiber surface modifications can also occur in acidic solutions via surface hydrolysis for hydrophilic features (Chand, 1982).

12.2.2 Surface modification by oxidation or reduction Fiber surface oxidation in aqueous solutions is an important process. About half of all textiles go through bleaching which is an oxidation process. Although whitening is the major purpose of the bleaching process, almost all bleached textile fibers have chemically changed surface properties. In general, a bleach process creates more functional chemical groups and gives the fabric better absorbent properties. A bleaching process also speeds up hydrolysis on the fiber surface and helps to remove impurities. Cellulose fibers such as cotton and rayon can be readily oxidized in an aqueous solution to create aldehyde, ketone and carboxylic acid groups. Since aldehyde

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O

OH

HO

O

O

HO

O

OH OH OH

[O] OH–

O O

HO

O

O

HO

O

OH OH OH

OH–

O–

O

O

HO

O HO

O

OH OH

OH

O O

HO

O– OH

+

HO

O O OH

Oxidative degradation on cellulose fibers

groups have strong reducing powers, the oxidative surface modification on cellulose fibers can be detected by the copper number; this is defined as the mass in grams of Cu(II) reduced to Cu(I) by 100 g of dry fibers with an alkali copper solution. The carboxylic acid groups can be measured by the exchange between a cation in solution and the solid cellulose substrate in its free acid form. Therefore a staining method with cationic dyes is often used to measure fabric surface damage on cotton fibers since the cationic dyes have a limited affinity to undamaged cellulose fibers. The greater the surface modification on the cellulose fibers, the deeper the color. Oxidative treatment often leads to cellulose degradations and therefore lower polymer molecular weights. The degradation can be evaluated by fluidity in cuprammonium hydroxide (CUAM) or fluidity in cupriethylene diamine hydroxide (CUEN), see AATCC TM 82 (AATCC, 2004). Although both CUAM and CUEN have been established as standard methods, the results are often too sensitive to testing conditions which leads to inaccurate readings. A nitration method, in which the cellulose is nitrated in a solution of nitric and phosphoric acids and phosphorus pentoxide, and dissolved in butyl acetate, is considered as a more accurate method for cellulose molecular weight measurement (Wakelyn et al., 2007).

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Hydrogen peroxide (H2O2) has achieved a dominant position as an oxidizing agent in textile wet processes owing to its environment-friendly nature and its relatively stable chemistry. It decomposes to oxygen and water after the applications and therefore no potential environmental issues arise. H2O2 can be readily activated by a catalyst or an activator during processing which makes it useful at lower temperatures. It is commonly used in aqueous alkali solutions with a stabilizer to slow down unwanted decomposition caused by transition metal ions. Sodium hypochlorite (NaClO) has lost ground to H2O2 in terms of its use in textile bleaching owing to increasing environmental concerns. However, it is still used broadly in the industry and even in consumer homes owing to its effectiveness at low temperatures. The NaClO action for fiber surface modifications is highly dependent on pH. The oxidation in the pH range 5–10 forms more carboxylic acid groups and less aldehyde and ketone groups compared with oxidation at pH values higher than 10. Bleaching with a high pH and a high temperature can cause rapid cellulose decomposition which leads to severe fiber damage (Hickman, 1995). Sodium chlorite (NaClO2) still has some applications in the industry owing to its advantages, i.e. it is the least sensitive to metallic contamination, it causes little chemical damage to fibers and no pre-scouring treatment is required. However, environmental issues, together with the higher cost relative to H2O2 and NaClO, have reduced its applications. Although fiber surface reduction in aqueous solution is not popular for cellulose fibers, it is very useful for some protein fibers such as wool. Peptide bond hydrolysis and reactions of the side chains of the amino acid residues are important in the surface modification of wool. The reactions of the cystine residues are of particular significance as the disulfide bond is very important in the overall performance of the wool fibers. The reactive character of the disulfide bonds is also the basis for many different textile processes. The cystine residues can be reduced by various reducing agents, including sulfite (SO32–), hydrosulfite (S2O42–), thiourea dioxide (H2NC(=NH)–SO2H), etc. Cysteine groups created at the reduction reactions can be used for many different surface modifications including grafting with other functional groups. P–CH2–S–S–CH2–P + HSO3– → P–CH2–SH + P–CH2–SSO3– The reducing agents can also help to remove iron contamination by reducing ferric (Fe3+) salts to ferrous (Fe2+) salts which are more readily complexed by chelating agents in a scouring and pre-H2O2 bleaching process.

12.2.3 Surface modification by active substance deposition Active substance deposition on textile surfaces is one of the most common approaches used in the surface modification of fabrics with aqueous solutions. In this approach, chemicals are added to the fabric surface at the end of the textile wet processes, i.e. the textile finish stage. The characteristics of the added chemicals

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have a large impact on fabric surface properties. The three most common chemical types are: chemicals that reduce the fabric friction coefficient to make the fabric smoother and softer; chemicals that make the fabric more hydrophilic to improve soil release and moisture management; and chemicals that make the fabric hydrophobic enough to repel stains such as coffee, wine and beverages. A fabric softening finish is the most popular textile wet process in the industry. The majority of fabric softeners stay on the fiber surface although some small softener molecules may penetrate into the fibers under certain conditions. In this case, fiber materials become more flexible as a result of the reduction in the glass transition temperature (Tg). Schindler and Hauser (2004) pointed out that the physical arrangement of the softener molecules on the fiber surface is important for their performance. Since most textile fibers carry negative charges in aqueous solution, the orientation of the softener on the fiber surface depends on its ionic nature. Cationic softeners orient themselves with their positively charged ends towards the partially negatively charged fiber, creating a new surface of hydrophobic alkyl chains that provides excellent softening and lubricity. Anionic softeners, on the other hand, orient themselves with their negatively charged ends repelled away from the negatively charged fiber surface. The orientation of nonionic softeners depends on the nature of the fiber surface. The hydrophilic portion of the softener is attracted to the fiber’s hydrophilic surface and the hydrophobic portion is attracted to the hydrophobic surface. Quaternary ammonium salts are commonly used cationic fabric softeners both in industry and in the home. They have the highest affinity to textile fibers in the batch process. Waxes are typically manufactured as emulsions in water, containing emulsifiers. Two major waxes are paraffin and polyethylene which can be produced in anionic, non-ionic or cationic forms, depending on the ionic nature of the emulsifiers. Silicones are increasing their importance in the textile softener market. Many different silicones have been developed as fabric softeners. Their applications depend on fiber types, application methods and the requirement for finish durability. Polydimethylsiloxane (PDMS) and amino functional siloxane are two major silicones used in the industry. A good performance during cleaning in home laundering is an important enduse property for apparel and home textiles. Since home laundering is carried out in alkali aqueous solutions, hydrophilic stains usually do not cause cleaning issues. For hydrophobic stains, such as fatty residues from the human body and oily food, cleaning is highly dependent on the nature of the fabric surface. It is very difficult to remove hydrophobic stains from a hydrophobic fabric surface in the home laundry. This situation is getting worse with the increasing use of hydrophobic polyester fibers and wrinkle-resistant finished cotton garments in the market place. A soil-release finish is a common approach to improving this situation. Adding hydrophilic chemicals to the fabric surface is the major approach for soil-release finishes. The finish increases fabric wettability and speeds up surfactant accessability and interactions with hydrophobic stains, leading to better

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cleaning. Carboxylic acid-based materials such as sodium carboxymethyl cellulose and hydroxyl-based materials such as starch are early approaches to creating soil-release finishes. Many different polyester–polyether copolymers have been developed as soil-release polymers. The copolymers may contain other chemical groups – such as carboxylic acids, amines, alcohols, etc. – to target different stains. This kind of soil-release polymer is particularly good for fabrics made from polyester fibers and blends. The polyester portion of the copolymer behaves like an anchor to stick on the polyester fiber surface and the hydrophilic polyether portion rises up to make the fabric more hydrophilic. The copolymers not only find broad applications in the textile industry, but also in detergents in consumer homes. In addition to applications as soil-release polymers, the copolymers are also being used to improve moisture management in performance wear. In order to make a fabric repel a stain, the fabric surface tension must be lower than the surface tension of the stain. The stain-repellent finishes achieve their performance by lowering the fabric surface energy by adding materials with a low surface energy. Therefore, different materials on the fabric surface may repel different stains. Paraffin wax, one of the earliest repellent agents used in the industry, can repel water but not oily stains. PDMS can repel aqueous stains and some oily stains owing to the fact that its surface energy is lower than that of the paraffin wax. Fluoro-polymer-based materials are the most popular stain-repellent agents used in the textile industry. They provide a fabric surface with the lowest energy of all repellent finishes and can repel both water and oil stains. With the correct choice of copolymer blocks, fluoro-polymers can achieve both repellency and soil-release functions (Sherman et al., 1969). These unique polymers have the unusual properties of being hydrophobic and oleophobic in air and having hydrophilic and soil-release properties during the laundering process. Most of the active sibstamces deposited on the fabric surface are not durable to home laundering, and could be washed away within five laundering cycles. Crosslinking agents can be used to increase the active substance durability by forming covalent bonds between the textile fibers and the active substances, particularly for natural fibers such as cotton, rayon and silk, etc. Commonly used cross-linking agents are those used in wrinkle-resistant finishes such as dimethylol dihydroxyl ethylene urea (DMDHEU) and its derivatives.

12.2.4 Surface modification by grafting Grafting in aqueous solutions is a useful approach to covalently bond functional chemicals on to a fiber surface to provide durable performance. Applications of grafting depend on the fiber structures, particularly the availability of different reactive groups. For cellulose fibers, abundant hydroxyl groups are available to react with many different functional chemicals to provide wrinkle resistance, stain repellency, flame resistance, and anti-microbial and soil-release properties, etc.

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Treatments based on condensation reactions and addition reactions are broadly used to produce etherified cellulose fibers. Carboxymethylation is a good example, whereby sodium carboxymethyl cellulose (CMC) is made via etherification reactions. The chemistry commonly used in wrinkle-resistant finishes is also an example of etherification reactions. Hydroxyl groups of cellulose fibers can also be readily reacted with carboxylic acids, acyl halides and anhydrides to form cellulose esters via esterification reactions. Non-formaldehyde wrinkle-resistant finishes using polycarboxylic acids such as butanetetracarboxylic acid (BTCA) on cotton fabrics are good examples of esterification reactions. Liu et al. (2005) reported on the surface modification of cotton fabric by grafting of polyurethane to improve fabric wrinkle resistance. Chemical modification of cotton was investigated using blocked isocyanates prepared from the reaction of 4, 4-diphenylmethane diisocyanate with poly(propylene glycol) followed by the addition of methyl ethyl ketoxime. For protein fibers such as wool and silk, primary and secondary amines are available for grafting reactions. Since amines are more reactive than hydroxyl groups, grafting on protein fibers can be applied at lower temperature than grafting on cellulose fibers. 1-Fluoro-2,4-dinitrobenzene (FDNB) is used to react with amines on wool and silk fibers for amino acid analysis. Since FDNB has a selective reactivity with reactive groups on protein fibers, it can be used to identify different amino acids. Hydroxyl, cysteine and carboxylic acids are also available for grafting with different functional chemicals. Some alkylating agents – such as iodoacetate ions, methyl iodide, N-ethylmaleimide and acrylonitrile – can react with cysteine residues to introduce different alkyl groups in wool fibers. For polyester fibers, surface hydrolysis is necessary to create enough hydroxyl and carboxylic acid groups for surface grafting. Li and Ding (2006) studied grafting of cellulose nano-crystals via reactions with hydroxyl groups at the surface of alkali-etched PET fabrics. Radical chemistry is widely used for surface grafting since it does not depend on the presence of reactive groups on textile fibers. Any fiber having CH bonds can be grafted via this technique. Polypropylene fibers have no reactive groups available for any of the grafting techniques used on cotton, silk, wool and polyester that were discussed above. However, grafting via radical chemistry is a useful approach for polypropylene fibers. Miyazaki et al. (1999) studied grafting modification of polypropylene fabric to give water-repellent and hygroscopic properties simultaneously. Cho et al. (2001) studied the fabrication of a deodorizing fabric by grafting of metal phthalocyanine derivatives onto polypropylene fabric. Cernakova et al. (2005) reported surface modification of polypropylene fabrics by acrylic acid grafting. Many different sources can be used to generate radicals available for the grafting, including plasma, ultraviolet (UV) light, initiators, etc. Many different polymers have been grafted on to almost all of the different textile fibers for various end-use properties.

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12.2.5 Strengths and weaknesses of different techniques Among the four major approaches for textile surface modifications in aqueous solutions discussed in Sections 12.2.1 to 12.2.4, acidic and alkali solutions are mainly used to recover the intrinsic surface properties of textile fibers. They are widely used in the textile preparation stage to get the textiles ready for other surface modifications in the dyeing, finishing and coating stages of textile processing. In some cases, acidic and alkali solutions are designed to achieve fiber surface hydrolysis to create new reactive groups that change the fibers’ intrinsic surface properties. A good example is using alkali solutions to change the polyester fiber surface from hydrophobic to hydrophilic for better anti-static and moisture management performance. This approach is necessary for the vast majority of textile fibers to make them useful in their end-use products, although a small amount of textile products may not go through surface modifications in acidic and alkali solutions. Textile fibers have different chemical structures and therefore specific pH and other conditions need to be carefully selected to prevent severe fiber damage. Cellulose fibers are relatively stable under alkali conditions. This is why most of the wet textile processes for cotton products are carried out in alkali solutions. This is also one of the major reasons why consumer detergents are formulated to be used in alkali conditions since cotton and cotton blends still dominate wash loads in most households. Oxidation and reduction in aqueous solutions create new chemical groups on the fiber surface. This approach is often used together with the approach described in Section 12.2.1 to make fibers more hydrophilic and more reactive to the chemistries used in the approaches described in Sections 12.2.3 (additives) and 12.2.4 (grafting). Although fiber surface oxidation or reduction may not lead to significant fiber damage, process conditions need to be carefully selected to limit the oxidation or reduction inside the fibers. Surface modifications by active substance deposition in aqueous solutions are broadly used in textile finishing stages to provide different end-use functions for textile products. Compared to the grafting approach in Section 12.2.4, active substance depositions have the advantages that: (a) a wide range of chemicals are available for almost all functions needed in textile products; (b) they are easy to use in the industry, using continuous processing for faster applications; (c) cost effectiveness is higher; (d) they have less sensitive requirements regarding process conditions and fiber properties. The major disadvantage for the approach described in Section 12.2.3 is that most active substance depositions are not durable to laundering, including commercial and home launderings. Although this weakness can be improved by using cross-linking, the chemistries used in the cross-linking process often: (a) require a slowing down of the process; (b) result in higher energy cost; and (c) have potential environmental issues since most crosslinking agents contain formaldehyde.

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12.3

Surface modification of textiles

Applications of surface modification of textiles by aqueous solutions

Surface modifications by aqueous solutions are possibly some of the oldest techniques applied in the textile field. Since most processes for pretreatment, dyeing and finishing of textiles are ‘wet processes’, it is simple and convenient to conduct the surface modification of textiles in aqueous solutions. In fact, most treatments in textile dyeing and finishing processes change the surface properties of fibers and therefore there are numerous examples of the surface modification of yarns, and woven and knitted fabrics, in aqueous solutions. In the sections below, examples have been selected that differ from the conventional processes and endow special functions or properties to textiles by modifying their surface. Six important processes dealing with different fibers are introduced under the following headings.

12.3.1 Mercerization for cotton fabrics Mercerization is named after John Mercer who, in 1850, discovered that cotton yarn or fabric exhibited swelling and shrinkage when immersed in a concentrated aqueous solution of caustic soda (Vigo, 1994). The mercerization of cotton has many beneficial effects including: • • • • • •

increased tensile strength; increased softness; increased luster (if mercerized under tension); improved affinity for dyes; improved dyeability of immature fibers; higher water sorption.

In mercerization, the chemical effect of concentrated caustic soda solutions is unusual because not only the amorphous but also the crystalline regions of the fiber are affected. Treatment of cellulose with caustic soda produces a polymorphic change in the crystalline structure of cellulose, i.e. conversion from cellulose I existing in a parallel chain conformation into cellulose II existing in an antiparallel chain conformation (Blackwell et al., 1980; Sakthivel, 1988; Turbak and Sakthivel, 1991). This change in the crystal structure is responsible for introducing unique surface properties to mercerized cotton fibers. The changes that occur in cotton upon mercerization depend on the concentration of the caustic soda used, the temperature of the treatment and whether or not the material is under tension during treatment. In order to reach the maximum fiber swelling and chemical penetration without fabric damage, the mercerization of cotton fabrics is frequently conducted at or below ambient temperatures with concentrations of NaOH at or greater than 20%. Swelling in caustic soda solution leads to a change of fiber cross-section from bean-shaped to

Surface modification of textiles by aqueous solutions

Before mercerization

After mercerization

Before mercerization

After mercerization

(a)

(b)

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12.1 Morphological changes of cotton fiber during mercerization: (a) before mercerization and (b) after mercerization.

round, and is accompanied by shrinkage in the length of the fibers (Fig. 12.1(a)). When the fabric to be mercerized is slack (mainly used for knitted cotton fabrics) without tension, shrinkage may occur in the lengthwise direction. When textiles are mercerized under tension (mainly used for woven cotton fabrics and yarns) the shrinkage can be minimized; at the same time the fiber surface becomes smoother. The high luster of mercerized cotton results from increased specular reflectance of incident light from the smooth fiber surfaces (Fig. 12.1(b)). Mercerization of yarns produces more dramatic changes in properties than mercerization of fabrics due to greater alkali penetration into the cellulosic fibers, causing greater conversion to cellulose II and changes in crystallite orientation (Zeronian et al., 1985). Mercerization of woven fabrics is usually done in a continuous process on machines with bowed rollers (chainless) or with stretching chains. The fabric is

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saturated with caustic soda solution in the padding and timing section first, and then passes through a tenter frame where the fabric is placed under tension in both the warp and filling directions. While still in the tenter frame, the fabric passes under cascades which wash away part of the caustic soda. Further washing is carried out when fabric is no longer under tension, with either hot or cold water. Finally, the fabric is neutralized with acid to remove the last traces of caustic soda. Knitted fabrics are usually mercerized in tubular machines or machines of related design that control and stabilize the shrinkage and tension of knitted fabrics to avoid the particular problems caused by a variation of the amount of shrinkage and tension from the edge to the center of the fabric when they are wet with alkali (Greenwood, 1987). In addition to conventional processes, techniques have been developed to mercerize cotton/rayon and cotton/polyester blends. Mercerization of rayon produces more pronounced swelling and shrinkage than mercerization of cotton owing to the lower degree of polymerization of the cellulose, while polyester fibers may be hydrolyzed at the fiber surface when immersed in alkali solutions (Freytag and Donze, 1983). Treatment with liquid ammonia produces changes in cotton similar to those produced by mercerization. However, the magnitude of the changes in cotton is not as great with liquid ammonia as with caustic soda mercerization (Gailey, 1970; Skaathun, 1970).

12.3.2 Moisture management finishes for performance apparel With the global trend of more active life styles, garment comfort during strenuous bodily activity and higher ambient temperatures becomes more and more important. Fabric moisture management capability is one of the key performance criteria in the performance apparel industry. Fabrics that rapidly transport moisture away from the human body make wearers feel more comfortable by keeping them dry. This action prevents perspiration from remaining next to the skin. In hot conditions, trapped moisture may heat up and lead to fatigue or diminished performance. In cold conditions, trapped moisture will drop in temperature and cause chilling and hypothermia. Excess moisture may also cause the garment to become heavy, as well as cause damage to the skin from chafing. For movement of liquid in a fabric, the liquid must wet the fabric surface before being transported by capillary action through the fabric pores formed between fibers and yarns (Kissa, 1996). This capillary action is determined by the interaction of the liquid and the fabric, by liquid properties such as viscosity and surface tension, and by the geometric structure of the pores. The size and the shape of the fibers, as well as their alignment, determine the geometry of the void spaces or pores through which the liquid is transported (Rajagopalan and Aneja, 2001). Quick wetting is a necessity and is the first step in wicking liquid away from

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human skin. Surface modification is often required to secure the quick wetting and wicking necessary for performance wear with good moisture management capacity. This surface modification in aqueous solutions is often called moisture management finishing. This hydrophilic finishing is one of the most effective ways to improve fabric wicking properties. The process facilitates quick absorption of moisture from the wearer’s body and promotes even distribution of moisture and speed of evaporation. The hydrophilic finishing can be classified into three categories: (a) the introduction of hydrophilic groups into polymer units of the fiber (for example, end-group modification by exposure to plasma or forming new groups by copolymerization); (b) the physical and/or chemical attachment of hydrophilic polymers to the fiber surface (coating, lamination or graft polymerization of hydrophilic materials); (c) the use of surfactants (including quaternary ammonium salts, detergents, fatty and sulfonic acids, special polymers and silicones) (Vigo, 1994). Any garment that is worn next to the skin or worn during exercise could be made of performance fabrics with moisture management properties. The range of applications for such fabrics continues to expand as new fabric technology is released on to the market. In addition to sportswear and active wear, there is also growing interest in moisture management fabrics from the routine apparel market. Fabric that can absorb and hold liquid moisture may slow down wicking rates and increase drying times, leading to the garment being less comfortable for the human body. Natural fibers such as cotton, silk and wool have high fiber swelling capacity which runs counter to the requirements for good wicking materials. This is why most performance wear is made from synthetic fibers. Without surface modifications, these fibers are often too hydrophobic to make a fabric with good moisture management properties. An excellent moisture management garment often has the following three characteristics: (a) it has poor absorbency and therefore does not hold on to too much liquid; (b) the fabric surface is hydrophilic enough to promote quick wetting and wicking; (c) the geometric structure of its pores or the void space between fibers and yarns leads to excellent capillary action. One example is a fabric created using CoolMax fiber, the four-channel polyester fiber from Invista; this has a special cross-section (in the shape of a cross) which creates ‘channels’ or ‘grooves’ on the fiber surface along the axes, and the spaces between fibers effectively form ‘tubes’. The fabric is surface modified to achieve an excellent wicking performance. An alternating copolymer of diethylene glycol terephthalate (hydrophobic) and polyethylene oxide (hydrophilic) has gained broad application in moisture management finishing. It has a molecular weight of around 30 000 and its polyethylene oxide segments have an average molecular weight of about 1000. This copolymer can be dispersed in water and applied to textiles by a padding– drying–curing process or a high temperature impregnation process. At the curing temperature of 160–170 ºC or dipping temperature of 130 ºC, the benzene rings in the terephthalate structures of the copolymer and in the polyester fiber move

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Water-/moistureabsorbent layer

Hydrophobic/waterpermeable layer Perspiration Vapor from perspiration

Perspiration Skin

12.2 Diagram of moisture management.

towards each other and combine together through the formation of inter-molecular structures (π electrons). Such combinations anchor the diethylene glycol terephthalate segments of the copolymer onto the surface of the fiber and endow the fiber with durable hydrophilic properties provided by the polyethylene oxide segments of the copolymer. The Nano-Dry finishing of Burlington can be mentioned as another example in this field: by means of this nanotechnology, molecular structures are changed through moisture. In this way, surface tension is changed thus resulting in quick moisture absorption. Moisture is extensively distributed on the fiber surface for quicker evaporation (Stegmaier et al., 2005).

12.3.3 Stain-repellent finishes for cotton and cotton blends Stain repellency is basically attained by limiting the wettability of fabrics. Cotton is a hydrophilic fiber that is more easily wetted by various liquid stains than hydrophobic fibers. Therefore, the stain-repellent finish is one of the most interesting new developments for cotton and cotton blends. Wetting is governed by surface and interfacial tensions. Thus, repellent finishes achieve their properties by reducing the surface energy of a fabric such that a substance does not spread on the fabric, making the fabric resistant to that substance.

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The oldest repellent finishes are those that repel water (Kissa, 1984; Singh, 1987). Drops of water should not spread on the surface of the water-repellentfinished textiles. Similarly, oil-repellent finishes should prevent oily fluids from wetting the treated textiles. Obviously, stain-repellent finishes should protect textiles from both water and oily stains. A good repellent finish modifies the surface of fibers and does not block the interstices. Therefore the air and water vapor permeability of the finished fabrics should not be significantly reduced (Friedrich and Schindler, 1990). This differs from waterproofing coatings of fabrics which have the disadvantages of stiff handle, lack of air and vapor permeability and consequently poor wear comfort. Repellents are usually divided into four distinct classes: paraffin wax, stearic acid–melamine, silicone and fluorocarbon-based polymers. Paraffin repellents were one of the earliest water repellents used in the industry, but they do not repel oily stains. Typically, the paraffin repellents are emulsions containing aluminium or zirconium salts of fatty acids. The paraffinic portion of the repellent mixture is attracted to the hydrophobic regions, while the polar ends of the fatty acid are attracted to the metal salts at the fiber surfaces. These finishes are relatively low cost, but the lack of oil repellency and poor durability limit the use of paraffinbased repellents. In stearic acid–melamine repellents, the hydrophobic groups of stearic acid provide the water repellency, while the remaining melamine groups can react with cellulose or with each other to provide permanent effects. The finished fabrics show an increased durability to laundering and a full hand, but decreased fabric tear strength and changes in shade of dyed fabrics; they also release formaldehyde which raises environmental concerns. Polymerized siloxanes are good water repellents. Dimethyl polysiloxane and methyl hydrogen polysiloxane, or a mixture of these two substances, are often used. In order to gain durability, silicones designed as water-repellent treatments usually consist of three components: a silanol, a silane and a catalyst such as tin octoate. The catalyst enables not only moderate condensation conditions, but also promotes the orientation of the silicone chains on the fiber surface. The outwardoriented methyl groups provide the water repellency. The most distinct advantages of silicone water repellents include a high degree of water repellency at low concentration based on weight of fabric, and very soft fabric hand. However, the oil repellency of silicone repellent finishes is very limited. Fluoro-chemical finishes impart both water repellency and oil repellency to fabrics, because fluorocarbon-based repellents provide fiber surfaces with the lowest surface energies of all repellent finishes in use. Most fluoro-chemical finishes use a pad–dry–cure process. Heat treatment is crucial for optimal repellency, since it provides an orientation of the perfluoro side chains to almost crystalline structures. The fabrics finished by fluorocarbon-based repellents are often referred to as being stain repellent or stain resistant. The excellent water repellency provided by the fluoro-chemicals impedes the access of the washing

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liquor during laundering. Therefore, so-called ‘dual-action fluoro-copolymers’ were developed. These copolymers combine repellency in the air and soil-release effects in aqueous solution (Sherman et al., 1969), leading to better removal of oily stains in domestic laundering. Of the four types of repellents discussed above, fluoro-chemical finishes dominate the current stain-repellent finish market with more than a 90% share. It is the only class of repellent that can repel oily stains and provide true stainrepellent textile products for the consumer. Although the unit chemical cost is higher for fluoro-chemical finishes than for the other three classes, it is much more effective considering the finish mass per weight of fabric. One big challenge to the use of fluoro-chemicals in stain-repellent finishes is increasing concerns about their impact on the environment and human safety. Although many short-chain fluoro-chemicals have been used in the industry to replace materials made from perfluorooctyl sulfonate, the concerns will continue until solid safety evidence is generated for these new fluoro-chemicals.

12.3.4 Fabric softeners and hand builders The way a fabric or garment feels to the touch is referred as its ‘hand’. Hand is important, and hand modification is one of the most common goals in textile finishing. Treatments that make the fabric more flexible and pliable impart the impression of softness. In general, softness comes from making the fibers themselves more flexible and from decreasing inter-fiber friction. Therefore, a material that can make textile fibers more flexible and lubricates the surface of fibers can be used as a softener. Softeners are among the most common textile chemicals. Most softeners consist of molecules with both a hydrophobic and a hydrophilic component. According to their ionic types and structures, softeners could be classified as cationic, anionic or non-ionic. These classes have different properties and each has its own advantages in particular cases. Most softeners have low water solubility and they are usually prepared as oil in water (O/W) emulsions containing 20–30% solids. Softening finishes are in fact a type of fiber surface modification since the softeners provide their main effects on the fiber surface. In finishing processes, softener molecules could be absorbed by the fiber and attached to the fiber surface in some particular way that depends on the ionic nature of the softener molecule and the relative hydrophobicity of the fiber surface. Cationic softeners – such as quaternary ammonium salts, amine salts, imidazolines and quaternaries with ester groups – orient themselves with their positively charged ends towards the partially negatively charged fiber (zeta potential), creating a new surface of hydrophobic carbon chains that provide the characteristic excellent softening and lubricity (Wakelyn and Johnson, 1972). They have inherent affinity for most fibers making them applicable using exhaust procedures. Anionic softeners, such as alkylsulfate salts and alkylsulfonate salts, orient themselves with their negatively charged ends

Surface modification of textiles by aqueous solutions (a)

(b) –

δ–

(c) –

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(d)

–

δ–

δ–

δ–

Hydrophobic part of softener molecule +

Cationic hydrophilic group

–

Anionic hydrophilic group Non-ionic hydrophilic group

δ–

Fiber surface with negative charge

12.3 Schematic orientation of softeners on the fiber surface. (a) Cationic softener and (b) anionic softener at fiber surface. Non-ionic softener at (c) hydrophilic and (d) hydrophobic fiber surface.

repelled away from the negatively charged fiber surface. This leads to higher hydrophilicity, but less softening than with cationic softeners. Anionic softeners do not have inherent affinity for most fibers and do not exhaust onto fabrics in batch processes. They are most suitable for application by padding. The orientation of non-ionic softeners – such as ethoxylated materials, silicones and hydrocarbon waxes – depends on the nature of the fiber surface, with the hydrophilic portion of the softener being attracted to hydrophilic surfaces and the hydrophobic portion being attracted to hydrophobic surfaces (Fig. 12.3). Silicone softeners have been the most widely used type of softener in recent years. The structures available are diverse, yet are all based on the same Si–O backbone. They provide very high softness, special unique hand, high lubricity, good sewability, elastic resilience, crease recovery, abrasion resistance and tear strength. They show good temperature stability and durability, with a high degree of performance for those products that form cross-linked films and a range of properties from hydrophobic to hydrophilic (Habereder, 1997; Thoss et al., 2003). Silicones containing a terminal hydroxyl or other reactive group can achieve better durability by using a silane catalyst which can cross-link the silicone chains to give a durable elastomeric structure. Aminofunctional silicones, the most

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commonly used type of silicone softeners, tend to be of a mildly cationic nature in acidic solutions because of the presence of the amino group and will therefore readily exhaust on to fiber surfaces. Hand building has the opposite effect to softening. Hand builders add stiffness to the fibers and fabric. Materials that make fibers stiffer, bind fibers together or increase inter-fiber friction can produce stiffer textile products. Hand-building finishes may be either temporary or durable. A temporary hand builder, can be washed out after a few laundry cycles, may be added to improve the sewing characteristics of a fabric or to add weight and make the fabric appear more substantial to the consumer. Starch, modified starch and polyvinyl alcohol are the most common hand builders for temporary hand building. Since they are soluble or dispersible in water, they can be applied by padding from aqueous solutions followed simply by drying. Vinyl acetate-containing polymers, acrylic copolymers and thermosetting polymers are more durable hand builders. They are usually aqueous solutions, emulsions or dispersions and can be applied to fabrics by a pad–dry–cure process. Thermosetting polymers are potential cross-linkers for cellulose fibers but function mainly by polymerization on the fiber surface during the curing stage of their application. Vinyl acetate-containing polymers and acrylic copolymers function by much the same mechanism as starch and polyvinyl alcohol but they can form water-insoluble films on the fiber surface and are therefore durable to washing.

12.3.5 Surface hydrolysis of polyester Polyester (PET) has been one of the most popular fibers in recent years. Polyester fibers have many desirable properties that make them excellent candidates not only for apparel and textile products but also for industrial and composite applications. However, the hydrophobicity of polyester fibers contributes to some less desirable properties – such as poor wetting and soil-release behavior in aqueous liquids, attraction to oily soils and rapid build up of static electricity. These undesirable properties have constrained their applications in the industry; much research effort has therefore been dedicated to making polyester fibers more hydrophilic to overcome these limitations. Polyester fibers can be modified by several different strategies (Lewin and Sello, 1984). Surface modification by alkaline hydrolysis is one of the most documented and widely used methods. The nucleophilic attack of a base on the electron-deficient carbonyl carbon in polyester causes chain scissions at the ester linkages along the polyester chain, producing carboxylic acid and hydroxyl polar end groups. The increased surface polarity leads to better wettability and soilrelease properties. With the progressive alkaline hydrolysis, the polyester chains on the fiber surface are etched away and the fiber diameter is reduced, producing fabrics with a softer and more silky hand. The mass loss from alkaline hydrolysis indicates the extent of hydrolysis and

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reduces fiber dimension. The hydrolysis of polyester groups initially causes (surface) chain shortening. As the chains become shorter, continued reaction will result in the loss of material and a reduction in fiber diameter, but the overall molecular weight remains largely unchanged due to the fact that these reactions only occur on the fiber surface. This is used commercially as the so-called ‘denier reduction’. The presence of a cationic surfactant has been shown to increase the activation energy and pre-exponential collision frequency factor. It is evident that surfactants affect the transport of the reactant at the polyester–water interface and their energy state. Alkaline hydrolysis of polyester can be controlled to alter fabrics to varying degrees, i.e. from surface hydrolysis to extensive removal of the constituent polymer. As hydrolysis progresses, fiber surfaces are etched away and alteration of the fabric pore structure with increased severity of hydrolysis is expected. By varying the extent of the hydrolysis, modification of the fiber surface wettability alone or improved surface wettability combined with altered fabric pore structure is possible. The mass loss, porosity and thickness reduction in polyester fabrics increases with increasing hydrolysis temperature or time. Optimal hydrolysis conditions for achieving the highest wettability in polyester fabrics could be a 3 N NaOH concentration and a temperature of 55 ºC. At higher hydrolysis temperatures, further enlargement of the fabric pore structure is observed with no further improvement in wetting. Wetting properties and pore structure vary considerably in polyester fabrics; however, for all polyester fabrics, water absorbency increases linearly with improved water wettability or increasing cosine water contact angles. Pore-size distribution and pore connectivities are also crucial to the improved water-retention properties (Hsieh, 2001).

12.3.6 Shrinkage control for wool The felting shrinkage of wool is primarily the result of the differential friction of wool cuticular scales, so the practical processes of shrinkage control in wool have their basis in surface modification. These processes can be divided into two distinct classes, namely: degradative treatments and additive polymer treatments. Degradative treatments The earliest degradative treatment was chlorination using hydrochloric acid during the latter half of the nineteenth century. Optical microscopy showed that the scales were severely damaged or removed completely. The treatment often damaged the wool, not only the cuticle, but also the cortex. Later a number of refined degradative treatments were developed using chemical, physical or biochemical techniques. The chemical degradative treatments included hypochlorite or dichlorodicyanuric acid (DCCA) wet chlorination (Bergen, 1970), alcoholic alkali treatment at low

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(a)

(b)

12.4 Images of the wool fiber before (a) and after (b) DCCA treatment.

water content, potassium permanganate/salt treatment, permonosulfuric acid treatment (Shao et al., 2001) and ozone treatment. The physical degradative treatments covered corona or plasma electrical discharges, while biochemical degradative treatments employed various enzymes (Guo et al., 2006). The difference in the wool fiber surface before and after DCCA treatment is shown in Fig. 12.4 The general mechanism of shrinkage control by a degradative process involves the softening of cuticular scales (Makinson, 1979). The softening is the result of oxidation and scission of the numerous disulfide bonds in the exocuticle of the wool. The formation of charged groups by oxidation and scission, particularly the formation of sulfonate groups (–SO3–) by the full oxidation of disulfide bonds in cystine residues increases the sorption of water, promoting cuticular swelling and softening. This softening causes an increase in the coefficient of with-scale friction and a decrease in the frictional difference. The increase in the coefficient of withscale friction makes it more difficult to move individual fibers in an assembly of fibers, while the decrease in the frictional difference reduces the unidirectional frictional movement of fibers. Both of these mechanisms contribute to the shrink resistance of degradatively treated wool. The direct repulsion of charged fibers may also play some part in the shrink resistance of degradatively treated wool. A successful degradative shrinkage control process requires that the exocuticle of the fiber is adequately softened without damage to the cortex of the wool fiber. Additive polymer treatments Degradative treatments confer shrink resistance to wool, but severe degradative treatments are liable to damage wool bulk, while mild degradative treatments only achieve low levels of shrink resistance. Polymer treatments were introduced to overcome the fiber damage problem inherent in degradative processes. However, polymer systems usually affect fabric handle more than degradative processes. Polymer treatments are based on the application of pre-formed polymers from

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aqueous dispersions to wool tops/fabrics by exhaust, pad–dry–cure or pad batch methods. In all successful shrinkage control treatments of this type, the polymer is deposited on the surface of the fibers. In certain systems the polymer contains reactive groups and is able to cross-link with wool fibers. There are three main mechanisms of shrinkage control by polymer treatments (Makinson, 1979). 1

2

3

Inter-fiber bonding. The polymer binds the fibers together at the points where they touch or come close together, thus preventing relative movement between the fibers. This primary mechanism is responsible for the shrinkage control effect of polymer systems applied to fabrics and yarns only. Scale masking. As an alternative to removing or softening the scale, polymer treatments can mask the scales by coating with a polymer film. Scale masking reduces the frictional difference of wool, and this mechanism is primarily advanced for shrinkage control in wool tops. Stand-off mechanism. The non-uniformly deposited polymer particles hold neighboring fibers slightly apart to prevent the scales from engaging with each other, instead of scale masking by forming a continuous polymer film. Using this mechanism, the polymer system can be applied at lower levels, and has a negligible effect on hand if suitable polymer particles are applied.

The chlorine/Hercosett process is a major shrinkage control process for wool both in top form and in fabric form. It is the combination of a chlorine degradative treatment with a Hercosett polymer treatment. Although the chlorine/Hercosett process is highly successful owing to its cheaper cost and practical effectiveness, it is now under pressure from environmental legislation, due to the generation of adsorbable organohalogens (AOX) during processing. Alternative surface modification techniques for shrinkage control in wool are currently being explored. A number of low-AOX or AOX-free processes have been developed but none are entirely satisfactory, so the development of an ideal environmentally acceptable low-AOX or AOX-free wool surface modification finish is still one of the principal aims of wool research.

12.4

Future trends

Since the surface modification of textiles in aqueous solutions touches almost all important sectors in the textile industry, surface modification research and the textile industry basically share the market trends and innovations. The increasing importance of polyester fibers is one of the biggest market trends in the global textile industry. Cotton remains the top apparel fiber worldwide with a 45% market share. However, it is expected that cotton will drop into second place behind polyester in 2012. Polyester gained market share from wool and other fibers from 1980 to 1990. Since 1990, polyester has begun to grab market share from cotton and is expected to continue this trend in the near future in spite of high oil prices (Textile Pipeline, 2008). This trend has boosted many new developments and

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60

Market share (%)

50 40

Cotton Wool Polyester Others

30 20 10 0 1980

1990

2000

2010

Year

12.5 Global apparel fiber market trends from 1980 to 2010.

innovations in the surface modification of polyester fibers. The surface-modified polyester fibers have established a dominant position in performance apparel or active apparel. Global consumers are moving to more active lifestyles, which is resulting in performance apparel becoming more and more popular in the global market (Fig. 12.5). Recent advances in information technology, biotechnology, nanotechnology and new materials have boosted textile innovations to meet increasing consumer demands. Many innovations involve the surface modification of textiles in aqueous solutions. One example is the application of nanotechnology in stainrepellent finishes to create the Lotus-Effect on fabric surfaces for enhanced performance, durability and cost effectiveness (Luzinov et al., 2006). The process includes attachment of a multitude of nano- and/or submicron-sized structures to a surface to provide increased surface roughness. In addition, the process includes grafting a hydrophobic material to the surface in order to decrease the surface energy and decrease the wettability of the surface. The combination of increased surface roughness and decreased surface energy can provide an ultrahydrophobic surface on the treated textiles. The recent developments of nanotechnology in textile applications are summarized by Qian and Hinestroza (2004). With advances in biotechnology, many enzymes have found applications in the textile industry and in consumer products for textile surface modifications (Schindler and Hauser, 2004). Cellulase has been broadly used for bio-polishing of denim to improve softness and appearance as well as for anti-pilling for knit fabrics, including in consumer detergents. Amylase, proteases and lipase have been used in industrial and consumer detergents that clean fabric surfaces by targeted hydrolysis of different impurities and stains. Peroxidase has been used during reactive dyeing to alleviate waste-water issues and improve color fastness.

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Esterase has been developed to clean polyester fibers and make them more hydrophilic for better moisture management and anti-static properties. It is clear that many more enzymes will be available in the near future for textile surface modifications in aqueous solutions.

12.5

References

AATCC (2004), ‘AATCC TM 82: Fluidity of dispersions of cellulose from bleached cotton cloth’, in Technical Manual of the American Association of Textile Chemists and Colorists, Research Triangle Park, North Carolina, AATCC, pp. 106–109. ABIDI N, HEQUET E, TARIMALA S AND DAI L L (2007), ‘Cotton fabric surface modification for improved UV radiation protection using sol-gel process’, Journal of Applied Polymer Science, 104(1), 111–117. BERGEN W V (1970), Wool Handbook, New York, John Wiley and Sons. BISHOP D P (1995), ‘Physical and chemical effects of domestic laundering processes’, in Carr C M (Ed.), Chemistry of the Textile Industry, London, Blackie Academic & Professional, pp. 125–172. BLACKWELL J, GARDNER K H, KOLPAK F J, MINK R AND CLAFFEY W B (1980), ‘Refinement of cellulose and chitin structures’, in French A and Gardner K H (Eds), Fiber Diffraction Methods, ACS Symposium series No. 141, Ch. 19, Washington, DC, American Chemical Society, pp. 315–334. CERNAKOVA L, KOVACIK D, ZAHORANOVA A, CERNAK M AND MAZUR M (2005), ‘Surface modification of polypropylene non-woven fabrics by atmospheric pressure plasma activation followed by acrylic acid grafting’, Plasma Chemistry and Plasma Processing, 25(4), 427–437. CHAND N (1982), ‘Surface studies of acid-treated poly(ethylene terephthalate) filaments’, Colloids and Surfaces, 4(2), 193–195. CHO D L, CHOI C N, KIM H J, KIM A K AND GO J H (2001), ‘Fabrication of deodorizing fabric by grafting phthalocyanine derivative onto nonwoven polypropylene fabric’, Journal of Applied Polymer Science, 82(4), 839–846. FREYTAG R AND DONZE J J (1983), ‘Alkali treatment of cellulose fibers’, in Lewin M and Sello S B (Eds), Handbook of Fiber Science and Technology: Vol. I. Chemical Processing of Fibers and Fabrics, Fundamentals and Preparation, New York, Marcel Decker, pp. 93–165. FRIEDRICH S AND SCHINDLER W (1990), ‘Influence and water and oil repellent finishing on the permeability to air of a woven cotton fabric’, Melliand Textilberichte, 71, 211–213, E67–E68. GAILEY R M (1970), ‘The liquid ammonia treatment of yarns and threads, 1. Principles and practice’, in Conference Proceedings on Liquid Ammonia Treatment of Cellulosic Textiles, Manchester, UK, Shirley Institute, pp. 9–20. GREENWOOD P F (1987), ‘Mercerization and liquid ammonia treatment of cotton’, Journal of the Society of Dyers and Colourists, 103, 342–349. GUO Y, LI X AND CAI Z (2006), ‘Wool shrinkage-resistance finishing by joint use of plasma and enzyme’, Maofang Keji, 2, 13–16. HABEREDER P (1997), Textilsilicone und Umwelt – fuer den Praktiker in Zahlen’, Melliand Textilberichte, 78, 352–354. HICKMAN W S (1995), ‘Preparation’ in Shore J (Ed.), Cellulosics Dyeing, Bradford, UK, Society of Dyers and Colourists, pp. 117–120.

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HOLMES S A AND ZERONIAN S H (1995), ‘Surface area of aqueous sodium hydroxide hydrolyzed high-speed spun poly(ethylene terephthalate) fibers’, Journal of Applied Polymer Science, 55(11), 1573–1581. HSIEH Y L (2001), ‘Surface characteristic of polyester fibers’, in Pastore C M and Kiekens P (Eds), Surface Characteristics of Fibers and Textiles, New York, Marcel Dekker, pp. 33–57. HYON S H (1998), ‘Surface morphology for poly-L-lactide fibers subjected to hydrolysis’, Sen’I Gakkaishi, 54(10) 527–531. ISODA H AND YASUDA H (2002), ‘Hydrophilic poly(lactic acid) fibers and their fabrics’, Japanese Patent 371463. KISHIDA Y, NISHIKAWA T, HONMA T AND TOMITA H (1987), ‘An electric conductive polyester product containing carbon-fibers and /or carbon-blacks’, Japanese Patent 62215635. KISSA E (1984), ‘Repellent finishes’, Handbook of Fiber Science and Technology, Vol. II, Chemical Processing of Fibers and Fabrics, Functional Finish, Part B, in Levin M and Sello S B (Eds), New York, Marcel Dekker, pp. 159–172. KISSA E (1996), ‘Wetting and wicking’, Textile Research Journal, 66(10), 660–668. KOTEK R, JUNG D W, KIM J H, SMITH B, GUZMAN P AND SCHMIDT B (2004), ‘Surface hydrolysis of filaments based on poly(trimethylene terephthalate) spun at high spinning speeds’, Journal of Applied Polymer Science, 92(3) 1724–1730. LEWIN M AND SELLO B (Eds) (1984), Handbook of Fiber Science and Technology, Vol. II, Chemical Processing of Fibers and Fabrics, Functional Finishes, Part B, New York, Marcel Dekker. LI W AND DING E (2006), ‘Characterization of PET fabrics surface modified by graft cellulose nano-crystal using TGA, FE-SEM and XPS’, Surface Review and Letters, 13(6), 819–823. LIU Y, HU J, ZHU Y AND YANG Z (2005), ‘Surface modification of cotton fabric by grafting of polyurethane’, Carbohydrate Polymers, 61(3), 276–280. LOBNIK A AND GUTMAHER A (2006), ‘Procedure for surface modification of nonwoven textiles with sol-gel siloxanes’, Slovenia Patent 21963. LUZINOV I A, BROWN P J, SWAMINATHA IYER K I, KLEP V Z AND ZDYRKO B V (2006), ‘Ultrahydrophobic substrates’, World patent WO 132694. MAKINSON K R (1979), Shrinkproofing of Wool, New York, Marcel Dekker. MIYAZAKI K, HISADA K, HORI T AND WATANABE N (1999), ‘Modification of polypropylene fabric for giving water repellent and hygroscopic properties simultaneously’, Sen’i Gakkaishi, 55(9), 408–415. MONTAZER M AND SADIGHI A (2006), ‘Optimization of the hot alkali treatment of polyester/ cotton fabric with sodium hydrosulfite’, Journal of Applied Polymer Science, 100(6), 5049–5055. NEEDLES H L, BROOK, D B AND KEIGHLEY J H (1995), ‘How alkali treatments affect selected properties of polyester, cotton and polyester/cotton fabrics’, Textile Chemist and Colorist, 17(9), 177–180. QIAN L AND HINESTROZA J P (2004), ‘Application of nanotechnology for high performance textiles’, Journal of Textile and Apparel, Technology and Management, 4(1), 1–7. RAJAGOPALAN D AND ANEJA A (2001), ‘Modeling capillary flow in complex geometries’, Textile Research Journal, 71(9), 813–821. REN X, WORLEY S D, KOU L, KOCER H B, ZHU C, BROUGHTON R M AND HUANG T S (2008), ‘Antimicrobial polyester’, in Abstracts of Papers, 235th ACS National Meeting, New Orleans, April 6–10, CELL 116.

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SAKTHIVEL A (1988), ‘Crystal structures of cellulose II derived from packing energy minimization’, Dissertation Abstracts International B, 49(5), 1744–1745. SCHINDLER W D AND HAUSER P J (2004), Chemical Finishing of Textiles, Boca Raton, Florida, CRC Press. SCHMALZ E, HUEBNER R, BENFER S, TOMARDL G AND BOEHM S (2003), ‘Surface modification of textile filtering materials by sol-gel procedure’, German Patent 10209667. SHAO J, LIU J AND CARR C M (2001), ‘Investigation into the synergistic effect between UV/ ozone exposure and peroxide pad-batch bleaching on printability of wool’, Coloration Technology, 117(5), 270–275. SHERMAN P O, SMITH S AND JOHANNESSEN B (1969), ‘Textile characteristics affecting the release of soil during laundering, Part II Fluorochemical soil release textile finishes’, Textile Research Journal, 39, 449. SINGH O P (1987), ‘Stain removal characteristics of fabrics and stain-resistance/release finishing’, Textile Dyer & Printer, 20, 24–27. SKAATHUN O (1970), ‘The liquid ammonia treatment of fabrics, 1. Development of process and plant’, in Conference Proceedings on Liquid Ammonia Treatment of Cellulosic Textiles, Manchester, UK, Shirley Institute. STEGMAIER T, MAVELY J AND SCHNEIDER P (2005), ‘High-performance and high-functional fibers and textiles’, in Shishoo R (Ed.), Textiles in sport, Cambridge, Woodhead Publishing Ltd, pp. 109–110. TEXTILE PIPELINE (2008), Special edition (Quarter 1), Crawley, UK, PCI Fibres. THOSS H, HESSE A, HOECKER H, WAGNER R AND LANGE H (2003), ‘Aufziehverhallten von ammoniummodifizierten Silikonweichmachern ayf CO-Geweber’, Melliand Textilberichte, 84, 314–318. TURBAK A AND SAKTHHIVEL A (1991), ‘Solving the cellulose enigmen’, Chetech, 20(7), 444–446. VIGO T L (1994), Textile Processing and Properties, New York, Elsevier, p. 37. WAKELYN P J AND JOHNSON R F (1972), ‘Orientation of antistatic agents at the surface of the acrylic fibers’, Journal of the Society of Dyers and Colourists, 88, 150. WAKELYN P J, BERTONIERE N R, FRENCH A D, THIBODEAUX D P, TRIPLETT B A, ROUSSELLE M A, GOYNES W R, EDWARDS J V, HUNTER L, MCALISTER D D AND GAMBLE G R (2007), ‘Cotton fibers’, in Lewin M (Ed.), Handbook of Fiber Chemistry, Third Edition, Boca Raton, Florida, CRC Press, pp. 600–603. YUAN X, MAK F T A AND HE B (2003), ‘Hydrolysis of poly(L-lactic acid) fibers and formation of low crystalline apatite on their surface by a biomimetic process’, Shengwu Yixue Gongchengxue Zazhi, 20(3), 404–407. ZERONIAN S H (1985), ‘Intracrystalline swelling of cellulose’, in Nevell T P and Zeronian S H (Eds), Cellulose Chemistry and its Applications, Chichester, Ellis Horwood Ltd, pp. 171–175.

13 Surface modification of textiles by plasma treatments R. R. MATHER

Heriot-Watt University, UK

Abstract: After a brief introduction, the nature of gas plasmas and methods for their generation are outlined. The relative merits and drawbacks of lowpressure and atmospheric-pressure plasma treatments are discussed, as are the strengths and limitations of plasma treatments compared with conventional treatments. Methods of characterising plasma-treated textiles surfaces are then surveyed. The modifications that plasmas can confer on textile surfaces are discussed in detail, and some representative applications of plasmatreated textiles are given. The chapter ends with some thoughts on the future scope of plasma treatments. Key words: plasma, plasma generation, plasma-treated textiles, surface functional groups, polymer grafting.

13.1

Introduction

Gas plasma treatments are set to revolutionise textile processing technology. Although gas plasmas have been known for several decades, it is only much more recently, since the introduction of equipment on an industrial scale, that commercial interest has begun to take hold. Indeed, a companion volume to this book, Plasma Technologies for Textiles, highlights the huge potential of plasma treatments for textile processing.1 The plasma treatment of textiles is attractive in that it is a clean, dry technology, which dispenses with the use of water or an organic solvent as a processing medium. Much less energy is consumed than in equivalent conventional treatments, and the scale of effluent is considerably reduced. In some cases, plasma treatments can bestow properties to textiles that are otherwise unobtainable. Plasma treatments of materials such as textiles alter their surface character without affecting their bulk properties. The depth of the surface treatment is <100 nm. The topography of the surface of the textile is modified, and its chemical properties may also be changed. Enhancements to the textile’s properties may include improved wettability or oil/water repellency, depending on whether greater chemical affinity or inertness has been conferred on the textile surfaces. Other properties that can be improved are adhesion, biocompatibility, resistance to 296

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wear and tear, rate and depth of dyeing, and cleaning of fibre surfaces, as in desizing. This chapter considers in detail the types of surface modification that gas plasma treatments confer on fibre surfaces. It is not the aim to provide detailed discussions of the various plasma technologies available or of the techniques available for characterising textile surfaces after plasma treatment, since extensive reviews of these topics have been written elsewhere.1 Only brief outlines of these topics will be given here.

13.2

Nature of plasmas

Gas plasmas are complex mixtures of photons, electrons, positive and negative ions, free radicals and excited molecules. The mixture of species composing a plasma is, therefore, highly complex, and in general all these species can interact with the textile surface. Striking everyday examples of plasmas include the solar corona, flashes of lightning and ‘neon’ lights (which are not always neon!). Plasmas are either thermal (hot) or non-thermal (cold). In a hot plasma, all the species composing the plasma are in thermodynamic equilibrium. Temperatures of the order of 1000 K are attained, so clearly hot plasmas are unsuitable for treating textiles – and nearly all other materials! Cold plasmas, on the other hand, are maintained at around room temperature, or slightly above it, and so can be successfully applied to textile materials. The electrons in a plasma acquire energies in the range 0.1–10 eV, much higher than the energies of the ions and molecules, so equilibrium between all the species is far from established. Energy is lost through radiation, through conduction to the surfaces of the plasma chamber and through various interactions and reactions within the plasma. Energy has, therefore, to be supplied continuously to the plasma from an external source, usually by means of an electrical discharge, either at low pressure in a vacuum chamber or in an atmospheric-pressure plasma device. Several types of power supply are used: low frequency (50–450 kHz), radiofrequency (13.56 or 27.12 MHz) and microwave (915 MHz or 2450 MHz).2

13.3

Plasma generation

13.3.1 Low-pressure plasmas Low-pressure plasma technology originated in the microelectronics industry. Although processing conditions for the microelectronics industry are quite different from those required for textiles, low-pressure plasma equipment has nevertheless been adapted and developed for treating textiles. Although it is generally more expensive than atmospheric-pressure equipment to install, it tends to provide a better uniformity of treatment to a textile fabric. In addition, it uses far less gas, a factor that is particularly important for expensive gases such as fluorocarbons.

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Low-pressure systems generally operate at pressures between 0.01 and 1.0 mbar (1–100 Pa), and a flow of gas is fed continuously into the plasma chamber. To generate the plasma, electrical power up to 5 kW may be necessary, depending on the treatment to be applied and the size of the chamber. The power is generated by means of a high voltage applied between positive and earthed electrodes, often in the radiofrequency range, or from a microwave source. Nowadays, textile fabrics of breadth >1 m can be successfully treated on a commercial scale with low-pressure plasmas, often with the use of roll-to-roll web treatment equipment. The technology is widely perceived as being confined to batch processing, although this perception has been vigorously challenged.3 A detailed account of low-pressure plasma processing technology is available elsewhere.3

13.3.2 Atmospheric-pressure plasmas Introduction Atmospheric-pressure plasmas for treating textiles fall into three distinct categories: corona discharge, dielectric barrier discharge (DBD) and atmosphericpressure glow discharge (APGD). Each of these is now briefly discussed in turn. Extensive descriptions of these types of plasma are provided elsewhere.4, 5 Corona dischrage Corona discharge is the oldest type of plasma treatment. Systems generating corona discharges consist of two electrodes of opposite charge, connected to a source that can provide up to c.10 kV and separated by a small gap, often c. 1 mm. Within the gap is the gas plasma. The geometries of the two electrodes are very different: one electrode is highly curved, as with a thin wire or the point of a needle, while the other is planar or nearly planar. The gas plasma that is generated discharges in a spray away from the highly curved electrode. Corona discharges are, however, generally too weak and too inhomogeneous for plasma treatments of textiles. The density of the plasma falls markedly with distance from the point of generation, a factor that accounts for the small gap between the two electrodes. Some fabrics will, therefore, be too thick to be treated by a corona discharge, and by no means all the fibres composing the fabric will be exposed to the plasma. Dielectric barrier discharge DBD methods use an arrangement of two parallel plate electrodes, separated by c. 1 cm, and with a potential difference of up to 20 kV between them. In order to

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prevent arcs between the electrodes, one or both plates are covered with a suitable dielectric, such as a ceramic or glass. The DBD is powered by an alternating current with a frequency of 1–20 kHz. There are two forms of DBD: filamentary and homogeneous.4 In filamentary discharges, there are a variety of discrete parallel filamentary channels running between the two electrodes. Hence, the plasma is not truly continuous, and the textile substrate is not uniformly treated. However, with suitable adjustment of process control parameters – such as the frequency of the power source, the distance between the electrodes and the composition of the gas plasma – a homogeneous DBD can be created, which gives rise to much more uniform treatment of the textile substrate. Homogeneous discharges are, therefore, preferred for treatment of textiles. Atmospheric-pressure glow discharge An APGD is generated by application of a low voltage, c. 200 V, across parallel plate electrodes, which are separated from each other by a few millimetres. The frequencies used are generally in the MHz range, i.e. radiofrequencies, and so are much higher than those used for other atmospheric discharges. In addition, the operating voltage is much lower. An APGD is uniform across the fabric being treated, and also relatively stable. Indeed, it has been argued that APGD has many of the benefits of low-pressure plasmas, but without the need for low-pressure generation.2 Moreover, no dielectric material is used to cover the electrodes. The discharge is usually generated in helium, which on a commercial scale becomes expensive to operate, unless the helium can be substantially recovered. Argon and nitrogen have also been used.

13.4

Low-pressure versus atmospheric-pressure treatments

The relative advantages of low-pressure and atmospheric-pressure treatments on textile substrates are still the subject of considerable debate. Both treatments have their merits and their drawbacks. Low-pressure equipment consumes more energy than atmospheric-pressure equipment, because of the pumping system required to achieve low pressure. On the other hand, low-pressure equipment uses much smaller quantities of gas, a particularly important consideration where expensive gases, such as fluorocarbons, are utilised. Low-pressure equipment generally provides a more controlled, uniform effect on a textile surface. With atmosphericpressure equipment, a uniform treatment is often more difficult to obtain, although APGD methods are now beginning to achieve good uniformity. Atmosphericpressure equipment can be utilised as part of an overall continuous process, whereas low-pressure equipment is more restricted to batch processing. However, continuous roll-to-roll systems can be built, with the unwind and rewind rolls

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located outside the low-pressure treatment chamber.3 This technique is limited, though, to thinner fabrics. Moreover, care has to be taken regarding the evaporation of water from fabrics, especially from those containing natural fibres. A cold trap has to be installed in the pumping system.3 Finally, there is much debate about the relative operational expense of low-pressure and atmospheric-pressure technologies. In practice, expenditure is very much governed by the actual use to which particular equipment is put.

13.5

Strengths and limitations of plasma treatments

As stated in Section 13.1, gas plasma treatment technology offers a clean, dry alternative to many conventional textile processing technologies. Gas plasma treatments are highly surface specific: they do not affect the bulk properties of the textile fibres. Plasma technology is environmentally friendly. Since no wet chemistry is involved, water consumption is negligible. Furthermore, the consumption of energy and chemicals is considerably lower, and waste treatment is substantially reduced. An enormous variety of plasma treatments is now available, and so the changes that can be brought to textile surface character are numerous. Indeed, new types of surface modification continue to be reported frequently. Plasma treatments are, therefore, capable of giving rise to innovative types of textile – and, moreover, on a commercial scale. There are, however, traditional processes that plasma treatments have not replaced, most notably dyeing, but nevertheless they can markedly reduce the volume of chemicals required and the concentration of any pollutant in the effluent. In the textile sector, however, plasma technology is still largely confined to technical textiles.6 It has been stated that the correct application of plasma treatments requires a good knowledge of the chemical and physical nature of plasmas, especially for treatments of a variety of different types of textile. However, a similar argument was at one time advanced concerning the application of synthetic dyes to textiles. A major problem with plasma treatment technology is, therefore, its infancy, in that many textile processors are still cautious about it. In addition, the range of machinery is arguably still limited, and greater flexibility is still required. However, this problem will be resolved over time. A particularly important issue resides in the durability of plasma treatments. Durability is dominated by the use to which the textile is put. Those applications, such as clothing, that are subjected to numerous cleaning or washing cycles, are the most demanding, especially when it is recalled that plasma surface treatments extend to <100 nm. Durability also has to be taken into account in relation to subsequent processing treatments.

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301

Characterisation of plasma-treated textile surfaces

13.6.1 Introduction The characterisation of a textile surface that has been treated with a gas plasma is likely to involve at least one of the following:7 surface topography, surface chemical composition, mechanical strength, reaction to external agents (liquids or ultraviolet radiation) and durability. Many techniques applied to textile surface characterisation, such as microscopy, involve highly localised analysis that would be confined to a small part of a single fibre. The information gained may not, therefore, be representative of the whole fibre, and even less so of the fabric of which the fibre is a constituent. In addition, inhomogeneities may be apparent if the plasma has not reached all fibre surfaces to the same extent, so that not all the component fibres have been equally treated. The techniques for comparing textile surfaces before and after plasma treatment can be divided into (a) physical and topographical, and (b) chemical. Each of these techniques is now discussed briefly in turn. A more detailed account is available in a companion volume.7

13.6.2 Physical and topographical techniques Scanning electron microscopy Physical and topographical changes to textile surfaces can be examined by a variety of microscopies. Scanning electron microscopy (SEM) is a technique that has been available commercially for over 40 years. It is capable of providing a great depth of field, and provides images of very high resolution at magnification of ×10 000, or even greater. In order to obtain such images, however, a very thin metallic coating (often gold) has to be applied to the textile sample, so that a path for the discharge of electrons is provided. Moreover, the sample has to be evacuated. The image derived from SEM is generally qualitative, although some software can be used to determine roughness parameters. Numerous examples of the application of SEM to characterise a wide variety of plasma-treated textile surfaces have been published. Examples include reports on polyethylene terephthalate (PET),8 nylon,9 acrylic,10 polypropylene,9, 11 cotton,12 linen,13 wool12 and silk.14 An important advance in SEM has been the development of environmental scanning electron microscopy (ESEM). The need for a metallic coating over the sample under examination is reduced, and indeed is often eliminated. ESEM can be used at much higher pressures than conventional SEM, and so the behaviour of liquids on the sample can then be observed. Thus, changes in wetting character as a result of plasma treatment can be directly followed by ESEM.15–17

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Atomic force microscopy Atomic force microscopy (AFM) is one of a family of scanning probe microscopies, and is by far the most utilised in the investigation of textile surfaces. AFM operates by scanning across the sample surface with a sharp tip, a few micrometres long, made from silicon nitride, at the end of a cantilever. The contours of the surface are mapped by measuring the forces generated between the sample and the tip, as the tip scans the sample surface. The probe is controlled by an intricate control system, and can operate very close to the sample surface indeed. An allied method, lateral force microscopy (LFM), can also be usefully applied to textiles. LFM measures lateral deflections of the tip. A major advantage of AFM is that it can image surfaces without the need for any surface pre-treatment process. Moreover, imaging can be effected in air, and even in water; no evacuation of the sample is required. Thus, the actions of liquids on textile surfaces, including those treated by plasmas, can be assessed by AFM. A variety of papers have appeared in which the use of AFM to examine plasmatreated textiles has been reported. Examples include PET,18 polypropylene,11, 17 wool,19 cotton20 and silk.14

13.6.3 Chemical techniques X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) is a quantitative technique for measuring the elemental composition of the surface of a material, and it also determines the binding states of the elements. XPS normally probes to a depth of 10 nm. However, because XPS is an ultra-high vacuum technique, the sample to be analysed has first to be evacuated. XPS has found extensive use in the investigation of textile surfaces, and this use is now spreading to the study of plasma-treated textiles. Plasma-treated textiles investigated by XPS include acrylics,10 polypropylene,16 wool12 and cotton.12, 21 Fourier transform infrared spectroscopy Fourier transform infrared (FTIR) spectroscopy is an analytical tool that is often used in conjunction with XPS, to provide information on the surface chemical effects of a plasma treatment. The presence (or absence!) of a variety of functional groups can be detected at the textile surface, and so specific bonding characteristics at the surface can be determined. This information thus complements that obtained by XPS. Examples of plasma-treated textiles investigated using FTIR are PET,22 wool23 and cotton.24

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13.6.4 Other approaches Although the techniques described above are the most utilised for investigating the surface character of plasma-treated textiles, other approaches have also been reported. In most cases, these approaches do not directly examine physical or chemical changes. For example, in a number of studies, changes in the contact angle of water on a textile have been followed, either by static or dynamic wetting tests.18, 22 There are examples too of the use of ESEM to examine the behaviour of tiny drops of water on individual plasma-treated fibres.17

13.7

Modifications to textile surfaces

13.7.1 Introduction The nature of the modification of a textile surface by plasma treatments is governed by a variety of factors: the composition of the gas, the type of textile, the power and frequency of the electrical supply, and the temperature and duration of the treatment. Nevertheless, despite the complexity of all these factors, several effects can be broadly identified. One is etching or cleaning of a textile surface. This process involves the removal of tiny amounts of material from the surface. It is associated with changes in surface texture and wetting properties. It appears that polymers containing oxygen groups are more sensitive to etching than polyolefins. Under equivalent conditions of plasma treatment, PET fibres are more prone to etching than nylon 6 fibres, which in turn are more prone than polypropylene fibres.25 Another effect is surface chemical modification by virtue of the introduction of particular chemical functional groups. The type of group depends very much on the nature of the gas plasma. These groups may confer improved wettability, biocompatibility and adhesion, or alternatively they may increase inertness at the surface. The introduction of reactive sites at the fibre surface allows polymers to be grafted to it. A third effect is plasma polymerisation or plasma-controlled vapour deposition, which enables the deposition of very thin films of polymers onto textile surfaces. These polymeric coatings possess highly cross-linked structures that, whilst difficult to characterise, can nevertheless confer valuable technical properties. However, as highlighted in Section 13.4, whatever surface effect is desired, care has to be taken to produce a uniform treatment, because of the structure of a textile fabric. The fabric tends to form a barrier against the gas plasma, whereby the effect of the plasma is more pronounced at the edge of the fabric than elsewhere on it.26 In order to achieve a uniform treatment, structural factors – such as fibre structure, fabric structure and pore structure – need to be taken into account.26 Pressure and treatment time are also important for achieving a uniform result.27 The conditions of plasma treatment must, therefore, be matched to the structure of the textile. One

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Table 13.1 Effects of some gas plasmas on textile surfaces Gas plasma

Effect

Argon Oxygen

Increased surface roughness Oxygen-containing groups on surface Increased hydrophilicity Fluoropolymer films on surface Increased hydrophobicity Nitrogen-containing groups on surface

Fluorocarbons Nitrogen

report even suggests that the pressure range, 1–100 mbar is the optimum pressure range for most textile structures,28 and indeed low-pressure technology does generally provide a more uniform treatment. As already noted, a variety of different gases are commonly applied, depending on the desired surface effect. A few examples are given in Table 13.1. It should be noted that, in addition to the surface conditions induced by the plasma itself, further modifications may occur from the subsequent exposure of the plasmatreated sample to the atmosphere. The following sections provide examples of the effects of plasma treatments on specific types of textile. Although much useful information has been gained from investigations of plasma treatments of equivalent non-textile substrates (especially synthetic polymer materials), the discussion is restricted to textiles themselves, for the sake of brevity. Where results from different workers contradict one another, the discrepancy is often likely to be attributable to differences in treatment conditions.

13.7.2 Protein textiles Wool The dominant protein fibre is wool, and indeed most plasma treatment studies on protein fibres have been conducted on wool. Of particular importance in this context is the complex structure of the wool fibre surface, known as the cuticle. The cuticle possesses a scale structure, which consists of a number of component structures. The outermost component structure is the epicuticle, which consists predominantly of fatty acids. The epicuticle is responsible for the hydrophobic character of wool. Immediately below the epicuticle is the exocuticle, which largely consists of the protein, keratin, in a highly cross-linked form. Below the exocuticle is the endocuticle, which also contains keratin, though in a less crosslinked form. The scale structure too is noteworthy in giving rise to a directional frictional effect (DFE). The friction in the tip to root direction, along the wool fibre is much greater than that in the opposite direction. This property is responsible for felting in wool.

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The wool fibre surface acts as a very effective barrier to many treatments, such as dyeing, printing and shrinkproofing. Before these treatments can be applied, the fibre surface has to be modified, usually by an oxidation process, which traditionally is effected by a chlorinating agent. A more environmentally acceptable alternative to chlorination is plasma treatment. It has been demonstrated that plasma treatment produces –OH, –C=O and –COOH functional groups on the wool fibre surface. Furthermore, the residues of the amino acid, cystine, present in the cuticular keratins are oxidised to sulfosulfonate (Bunte salt) and cysteic acid moieties: >CH–CH2–S–S–CH2–CH< → >CH–CH2–S–SO3H >CH–CH2–SO3H The extent of oxidation strongly depends on the type of plasma treatment applied.29 The complex physical nature of the wool fibre surface is considerably modified by plasma treatments. The epicuticle is often completely abraded, with the consequent loss from the fibre of its fatty acid shield.30 Moreover, XPS analysis reveals a reduction in the content of aliphatic carbon.31 In addition, part of the exocuticle, the so-called ‘A-layer’, is also degraded, and it is here that the cysteic acid and Bunte salt moieties are formed. The removal of the outer layer of fatty acid and the appearance of a variety of oxygen-containing functional groups render plasma-treated wool hydrophilic, rather than hydrophobic. This change, along with the physical changes to the surface, make wool fibres much more easily wetted by aqueous media, such as dye solutions. The friction between adjacent fibres is also increased,31 although the difference in friction in the two opposing directions (DFE) is considerably reduced. The propensity for felting is, consequently, also significantly diminished.30 A full account of the plasma modification of wool is available elsewhere.31 Silk Raw silk, as secreted from the silk-worm, consists of pairs of fibroin filaments joined by a sericin gum. Both fibroin and sericin are proteins. Degumming, the removal of sericin, brings out the soft qualities of silk, and is often carried out by a soaping process that can take many hours. On degumming, silk fibres have the appearance of structureless threads, but in fact are composed of a large number of very fine fibrils. There are far fewer studies reporting on the plasma treatment of silk than there are for wool. Air and oxygen plasmas have been successfully applied to degumming, and indeed it appears that sericin can be removed much more quickly than in conventional degumming processes.6 A number of studies have concentrated on the use of sulfur hexafluoride (SF6) plasma (and also to some extent fluorocarbon plasmas), to confer hydrophobicity on silk fibres.14, 32, 33 Sulfur hexafluoride is used

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to avoid plasma polymerisation. It has been reported that the contact angles of water on treated silk fabrics can become as high as 130–140º, and in some cases higher still, even after quite brief treatment times. The conditions of treatment are highly influential on the level of hydrophobicity attained. XPS analysis has shown that, not surprisingly, the greatest hydrophobicity is achieved when the ratio of fluorine to carbon at the fibre surface has increased.14 In addition, it was stated in one report that the proportion of surface oxygen also increased after treatment with sulfur hexafluoride plasma,33 while another report has commented on the slight decrease in the ratio of oxygen to carbon.14 It is noteworthy too that increase in hydrophobicity is accompanied by an increase in crystallinity at the silk fibre surface33 and in surface roughness (from c. 10 to 30 nm).14 It appears then that, when fluorine atoms are implanted into the fibre surface structure, there is simultaneous etching of the surface amorphous regions. The changes in surface morphology, combined with the fluorinated nature of the surface, give rise to the high contact angles observed. The durability conferred by the treatment is also apparently greater.33

13.7.3 Cellulosic textiles Cotton Cotton fibres have a twisted ribbon-like appearance. They are structurally divided into concentric zones, with a hollow central core called the lumen. The thin outermost layer of each fibre is the cuticle, which primarily consists of pectin, fats and waxes. Immediately beneath the cuticle is a primary wall of cellulose, consisting of fibrils arranged in a criss-cross pattern. The secondary wall constitutes the main body of the fibre and consists very largely of cellulosic chains arranged in helical orientations. An important part of cotton processing is the removal of the fats and wax by scouring, in order to render the fibres hydrophilic. However, to the author’s knowledge, no plasma process has been reported for the removal of fats and wax. As with other cellulosic materials, the functionality of cotton fibres is limited to hydroxyl (–OH) groups, and so hydroxyl groups dominate the chemistry of cotton. However, plasma treatments allow the introduction of other surface functional groups, which can broaden the surface properties of the fibres considerably. Thus, XPS has revealed that treatment with oxygen plasma gives rise to surface carbonyl, ether and carboxylic groups,12, 34 and there is evidence too for some cross-linking of surface cellulose.12 These changes give rise, as would be expected, to greater hydrophilicity.24 Argon plasma treatments also result in fibre surface oxidation.12 The treatment itself produces free radicals at the surface, which then initiate oxidation when the sample is subsequently exposed to air. Both types of plasma treatment cause surface roughening.

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There has been some interest too in rendering cotton more hydrophobic. Hydrophobicity is appreciably enhanced on exposure of cotton to either tetrafluoromethane (CF4) plasma or to hexafluoropropene (C3F6) plasma.21 In both cases, fluoropolymers are generally formed on the surface, although hexafluoropropene produces the higher hydrophobicity. It is suggested that, whereas tetrafluoromethane plasma can generate surface polymers through plasma polymerisation, hexafluoropropene can additionally generate further polymers by inducing other polymerisation mechanisms. Furthermore, tetrafluoromethane generates more atomic fluorine, which can abrade the fluoropolymers that have been deposited. Sulfur hexafluoride (SF6) plasma also renders cotton more hydrophobic.35 It has been observed too that some oxygen-containing groups are formed after treatment with a fluorinated gas plasma. This effect occurs on subsequent uptake of atmospheric oxygen. The effectiveness of air–helium and air–oxygen–helium plasma treatments for desizing polyvinyl alcohol (PVA) on cotton fabrics has also been demonstrated, with the latter plasma more effective.36 XPS investigations of the treated PVA films revealed some scission of the PVA chains and the formation of polar groups. The solubility of the PVA in cold water was thus increased. Linen A few reports have been published on plasma treatments of linen. As with cotton, argon and oxygen plasma treatments give rise to increases in surface oxygen content, and also to the formation of voids and cracks on the fibre surface.13 With regard to surface crystallinity, there is either no significant change13 or some increase,37 no doubt depending on the treatment conditions applied. Viscose Little appears to have been published on plasma treatments of viscose rayon. However, some evidence has been presented that argon plasma causes the formation of a variety of polar groups, with a consequent increase in wettability.38 Crystallinity is reduced, and some damage is apparently caused to the cellulosic chains. A report on air–helium and air–oxygen–helium plasma treatments for desizing PVA on viscose fabrics has also been published.39 As with the desizing of cotton fabrics, both plasma treatments give rise to an increase in the solubility of the PVA size in cold water.

13.7.4 Synthetic textiles Introduction As synthetic textiles cover such a wide range, both in terms of their polymeric constitution and their application, only a brief summary of the effects of plasma

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treatments on them can be given here. The synthetic textiles highlighted in this section are commodity textiles for which a wide range of information is now available regarding the effects on them of plasma treatments. However, it will become apparent that, in broad terms, the types of plasma treatment already described for natural fibres produce many similar effects in synthetic textiles. A fuller account can be accessed elsewhere.6 Polypropylene Despite the wide range of applications that polypropylene textiles enjoy, their low surface energy and consequent hydrophobicity can, nevertheless, be restrictive. For this reason and because of their smooth fibre surfaces, polypropylene textiles are difficult to coat by conventional means. However, plasma technology can provide a means of overcoming this difficulty, through pitting of the fibre surfaces and changes to their chemical nature. Moreover, hydrophobicity is a disadvantage for any application requiring good interactions with aqueous environments, but suitable plasma treatments, such as with oxygen and argon plasmas, render polypropylene textiles hydrophilic.16, 17 It appears that alcohol, carbonyl, ester and ether groups are created, and maybe carbon–carbon double bonds as well.40, 41 Nitrogen-containing plasmas produce surface amine (–NH2), imine (–CH=NH) and cyano (–CN) groups.6 The hydrophilicity induced by oxygen and argon plasmas gives rise to a sharp reduction in the contact angle of water droplets deposited on polypropylene fibre surfaces.40, 41 Because the fibre surfaces are simultaneously roughened by the treatments, the droplets become flattened in a direction at right angles to the fibre axis.17 Polypropylene textile surfaces may also be made more oleophobic. This increased resistance to oils can be useful in some filtration systems. Increased hydrophobicity can be conferred by plasma treatment with a fluorocarbon, so that a fluoropolymer is deposited at the fibre surface. One report has revealed that a variety of fluorinated functional groups are present, including –CHF, –CF2–, –CF3 and –CH2–CF2–.42 There have been questions about the durability of some plasma treatments on polypropylene textiles.25 There is evidence that the outer polymer layers of oxygen-treated fibres consist of oxidised and unoxidised segments, and that these layers rearrange themselves over time, in order to minimise surface energy. The type and extent of rearrangement can be considerably reduced by prior treatment of the polypropylene fibres with helium or argon plasma, through cross-linking of the polymer chains at the surface, a treatment known as CASING (cross-linking by activated species of inert gases).25

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Polyethylene terephthalate (PET) Plasma treatment of PET textiles has also been a subject of considerable interest, and a variety of effects have been observed. For example, treatment with air plasma at low pressure increases the hydrophilicity of PET textiles, but there may then be a progressive reduction in hydrophilicity over the succeeding weeks after treatment.43 Air plasma treatment also raises the oxygen content of the PET fibre surface and introduces nitrogen to it. It is suggested that hydroxyl groups, and possibly amine groups, are created. However, XPS results have shown that after plasma treatment, although there is no marked variation in the level of surface nitrogen, the level of oxygen steadily declines. This reduction explains the decrease observed in hydrophilicity. During plasma treatment, surface amide (–CO–NH–) groups are also formed, and their level does not subsequently decrease. Morphological changes were also noted after air plasma treatment. Whereas the surface of the untreated fibres contained fibrils oriented in the direction of each fibre, air plasma treatment gave rise to a more uniform, pitted surface structure. The density and depth of these pits have been observed to rise to a plateau as a function of exposure time.44 Roughness also increases as a function of plasma pressure. These etching effects are also apparent after treatment with helium and argon plasmas. Treatment with oxygen plasma has also been reported.18 A steady increase in the roughness of the fibre surfaces has again been observed, as has the appearance of surface carbonyl groups. Investigations on the effects of treatment with sulfur hexafluoride and fluorocarbon plasmas have been conducted. At very low plasma pressures, an increase in surface roughness is still noted, but further increases in pressure lead to a steady decrease in roughness.44 As would be expected, the treatments also give rise to increased hydrophobicity, and XPS showed the presence of fluorinated aliphatic carbons and also some isolated carbonyl groups.35 Some surface oxidation was also observed, when the PET textile was subsequently exposed to atmospheric oxygen. One interesting observation was the high, stable hydrophobicity attained after repeated cycles of sulfur hexafluoride plasma treatments followed by washing treatments.35 A report has been published too on the treatment of PET fabrics with tetrachlorosilane (SiCl4) at atmospheric pressure.45 The study confirmed the presence of silicon-containing groups on the fabric surface. After exposure of the fabric to the atmosphere, most of the Si–Cl bonds produced were transformed to Si–OH groups. The presence of these groups and the roughened surface enhanced the hydrophilicity of the fabrics.

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Nylon 6 and 66 Nylon textiles appear to have attracted less interest than PET textiles. Nevertheless, several studies still deserve mention. For example, it has been reported that after brief treatments of oxygen or argon plasma in a glow discharge generator, the surfaces of nylon 6 fibres actually become smoother, although longer treatments give rise to ripple-like patterns, oriented at right angles to the fibre axis.46 Oxygen plasma gives the more distinct effects. A short treatment with tetrafluoromethane plasma gives rise to some granular patches and the appearance of a thin film. After longer treatments, only ablation and ripple-like patterns are observed. Grainy surfaces have also been observed on nylon 66 textiles, treated with helium and helium–oxygen plasmas.9 XPS has revealed a slight increase in surface oxygen content after these treatments. An interesting study has been undertaken of the action of oxygen plasma on nylon 6 nanofibres at low pressure.47 Additional oxygen-containing groups are created, such as hydroxyl and carboxylic acid groups. Whereas the untreated nanofibres possessed smooth surfaces but an uneven cross-section, the surfaces of the treated nanofibres appeared to be composed of pores and aggregates. After longer treatments, grooves could also be observed etched on the nanofibre surfaces. In addition, whereas water droplets sit on the untreated nanofibres with a contact angle of a little less than 90º, on the treated nanofibres the water appears to form thin films. Hence, the contact angle is considerably reduced.

13.7.5 Grafting An important advantage offered by plasma treatments is the scope for grafting polymers onto textile surfaces. As is evident in Section 13.8, grafting allows the nature of the surfaces to be tailored to better suit the application for which the textile is intended. In addition, control of the conditions for grafting can determine the level of hydrophilicity imparted to the textile.17 Polymer grafting to numerous types of fibre has been reported. These include not only synthetic fibres, but also natural fibres such as cotton and linen.48–50 The commonest approach is first to treat the textile with a gas plasma, such as argon or oxygen plasma, which will not promote plasma polymerisation on the textile surface. After the plasma treatment, the textile is exposed to a vinyl monomer. The plasma treatment creates active sites on the textile surface, so that the monomer is polymerised. Exposure of the plasma-treated textile to air for several minutes before the polymer grafting stage can encourage the additional formation of surface peroxide, in order to facilitate polymer grafting.51 Vinyl monomers often used for grafting include acrylic acid, acrylamide and acrylonitrile.

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13.8

311

Applications

13.8.1 Textile dyeing The application of dyes to textiles can be enhanced after suitable plasma pretreatments. Barriers to the diffusion of dyes into the bulk of the fibres may be considerably eased, so that the rate of dyeing is increased. The presence of more suitable functional groups may also improve dyeing characteristics. However, plasma treatment conditions may at the same time increase the overall surface crystallinity, by preferentially etching the amorphous surface regions. This factor militates against improved dyeing, and has to be taken into account, especially with synthetic textiles. There are a number of reports on the application of dyes to plasma-treated wool. As noted in Section 13.7.2 (subsection on ‘Wool’), the wool fibre surface is severely modified by plasma treatments, which consequently allow much easier ingress of dye. In a number of cases, an increased, and even more uniform uptake of dye has also been observed.31 In particular, it has been noted that air and nitrogen plasmas produce significant effects in the application of acid dyes.6 The increase in dye uptake may be attributable to the creation of more amine groups in the keratin chains. However, it has also been reported that overall dye uptake is hardly changed after nitrogen plasma treatment, despite an increased rate of dyeing.29 Improvements in cotton dyeing can also be effected by a suitable plasma treatment.24 It has been reported, for example, that uptake of the direct dye, Chloramine Fast Red K (CI Direct Red 81), on cotton fabrics increased when the fabrics had first been treated with oxygen plasma.52 These treatments promoted fibre surface erosion and increased the surface content of carbonyl and carboxylic acid groups. However, the fabric was observed to age after plasma treatment, with loss of the improved dye uptake. Improvements in ink-jet printing of cotton fabrics after plasma treatment has also been observed.53 The dyeing characteristics of synthetic textiles can also be enhanced by a prior plasma treatment,9, 22 although care has to be taken with the treatment conditions. As with textiles composed of natural fibres, the surface structure of the fibres in a synthetic textile can be highly influential on successful dye application.54, 55 It has been reported that on the surfaces of PET and nylon 66 fibres, the most affected parts are the non-crystalline domains. These are the most accessible regions for the dye. A plasma treatment, therefore, which etches the non-crystalline domains at the fibre surface to a much greater extent than the more crystalline regions, may instead give rise to impaired dyeing characterisitics.

13.8.2 Shrinkproofing of wool In most conventional wool shrinkproofing processes, the wool fibres are first submitted to chlorination, before being coated with a suitable polymer. This is the

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basis, for example, of the widely used chlorine–Hercosett process. However, as stated in Section 13.7.2 (subsection on ‘Wool’), a more environmentally acceptable alternative to fibre surface oxidation by a chlorinating agent is oxidation using a suitable plasma treatment. Chlorination leads to the presence of adsorbable organic halides (AOX) in the resulting effluent, and the generation of AOX during conventional shrinkproofing often exceeds the maximum levels permitted by legislation. Surface oxidation induced by plasma treatment prior to wool fibre coating by plasma polymerisation reduces, rather than eliminates, wool fibre shrinkage and consequent felting. Moreover, plasma polymers deposited on wool reduce the uptake of the dye by wool.31 Shrinkproofing of wool by plasma treatment has, therefore, focused on surface oxidation, with new resins being formulated for subsequently coating the treated fibres.31

13.8.3 Biomedical textiles Textiles feature prominently in biomedical applications.56 A few examples are given in Table 13.2. Plasma treatments of biomedical textiles offer a number of potential advantages, most of which have yet to be fully exploited. One clear application is sterilisation, the inactivation of infectious micro-organisms that may be present on the fibre surfaces. Depending on the type of plasma treatment adopted, the mortality rates of the various micro-organisms exposed to the plasma will be different, so the effectiveness of a particular plasma sterilisation regime has to be closely assessed. Moreover, some micro-organisms may, through mutation, eventually become resistant to the regime, so a change in plasma treatment conditions would then have to be applied. The changes in surface chemical character that plasmas induce on a textile can also be exploited. It has already been noted that plasma treatment can create a variety of polar functional groups, such as oxygen- and nitrogen-containing groups. These groups can act as anchors for an enormous range of biological molecules, and so render a textile more biocompatible. Thus, polypropylene textiles, which find uses as sutures and hernia patches, are innately hydrophobic. Treatment with an air or oxygen plasma renders the fibre surfaces hydrophilic, which for many biological applications will be advantageous. Grafting of polymers has also been used to achieve surface functionalisations for specific biomedical uses. For example, after grafting of acrylic acid to PET or polypropylene fibre surfaces, chitosan and quaternised chitosan can then be attached to the bound polyacrylic acid through amide links.57, 58 The immobilised chitosan then confers antibacterial properties on the textile.

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Table 13.2 Textiles in biomedical applications Textile

Biomedical applications

Silk Cotton Viscose Polypropylene PET

Wound dressings, sutures, artificial tendons Wound dressings, bandages Wound dressings, bandages, artificial kidneys and livers Orthopaedic bandages, sutures, mechanical lungs Cardiovascular implants, sutures, orthopaedic bandages, artificial tendons and ligaments, artificial kidneys Wound dressings, compression bandages, sutures, surgical hosiery

Nylon

13.8.4 Oil and water repellency Water-repellent finishes are generally required in textiles manufactured for outdoor use, such as tents and outdoor clothing. Traditionally, suitable finishes can be applied by a variety of means, but all these finishes are applied from the liquid state. Oil-repellent finishes are increasingly being required too. Fluorinecontaining compounds are used to confer oil repellency, but again these compounds are applied from liquid media. Treatment with plasmas of fluorine compounds, such as sulfur hexafluoride or fluoro-compounds, provides an alternative means of conferring oil and water repellency to textiles, although it is important that the durability of the treatment matches or surpasses conventional repellent treatments. Fluorocarbons can create fluoropolymer coatings on the textile surface, but these coatings are susceptible to peeling away. Anchoring of these coatings would be stronger on fibre surfaces that have already been considerably roughened by the treatment. Treatments with sulfur hexafluoride, as noted above, introduce fluorine atoms to the polymer chains already present at the fibre surfaces. A detailed account of plasma treatments for oil and water repellency is available elsewhere.59

13.9

Future trends

The technology of treating textiles with gas plasmas is still only in its infancy. In view of its ecological and economic advantages, however, the technology is progressively gaining ground in the textile sector. With this technology, a wide range of surface properties can be tailored that are unattainable – or at best, attainable with difficulty – by conventional means. Section 13.8 has highlighted some textile applications that benefit from plasma technology, but they serve as just a few examples. Plasma technology, especially with subsequent grafting of suitable polymers to the textile, can promote anti-static properties, electrical conductivity and flame-retardant properties. It can enhance abrasion resistance,

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and also yarn strength, through increased attraction between the constituent fibres. Fibre surface properties can be adapted with a given textile end use in mind, without change to the textile’s bulk properties. A particular doubt voiced by textile processors concerns the durability of the treatments, in view of the very thin layer of fibre surface affected. Of course, the level of durability required depends on the desired end use of the textile. However, the CASING technique, described in Section 4.7.4 (subsection on ‘Polypropylene’), provides one approach to stabilising the fibre surface. Polymers grafted to fibre surfaces can also stabilise them. Finally, there is increasing – and welcome – interaction between textile technologists and designers, especially in the technical textiles sector. Plasma technology could well assist this dialogue. Changes to such characteristics as abrasion resistance and wetting properties will have a bearing on aesthetic properties, and so could be exploited in textile design, notably with respect to fabric handle and colour tone.60

13.10

Sources of further information and advice

In view of the attractions of plasma technology for the textile industry, a number of articles are available for further information. In addition to reference 1, reference 25 provides a useful review. Other sources of information include articles written by Ian Holme in the May 2003 and May 2005 issues of International Dyer and by J.-Y. Kang and M. Sarmadi in the October and November 2004 issues of AATCC Review.

13.11

References

1 SHISHOO R (Ed.) (2007), Plasma Technologies for Textiles, Woodhead Publishing Ltd, Cambridge. 2 SHISHOO R (2007), ‘Introduction – the potential of plasma technology in the textile industry’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. xv–xxx. 3 LIPPENS P (2007), ‘Low-pressure cold plasma technology’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. 64–78. 4 HERBERT T (2007), ‘Atmospheric-pressure cold plasma processing technology’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. 79–128. 5 STEGMAIER T, DINKELMANN A, VON ARNIM V AND RAU A (2007), ‘Corona and dielectric barrier discharge plasma treatment of textiles for technical applications’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. 129–157. 6 MARCANDALLI B AND RICCARDI C (2007), ‘Plasma treatments of fibres and textiles’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. 282–300. 7 NEVILLE A, MATHER R R AND WILSON J I B (2007), ‘Characterisation of plasma-treated

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26 VERSCHUREN J AND KIEKENS P (2005), ‘Gas flow around and through textile structures during plasma treatment’, AUTEX Res J, 5, 154–161. 27 DE GEYTER N, MORENT R AND LEYS C (2006), ‘Penetration of a dielectric barrier discharge plasma into textile structures at medium pressure’, Plasma Sources Sci Technol, 15, 78–84. 28 POLL H U, SCHLADITZ U AND SCHREITER S (2001), ‘Penetration of plasma effects into textile structures’, Surf Coat Technol, 142–144, 489–493. 29 KAN C W, CHAN K, YUEN C W M AND MIAO M H (1999), ‘Low temperature plasma on wool substrates: the effect of the nature of the gas’, Text Res J, 69, 407–416. 30 HESSE A, THOMAS H AND HÖCKER H (1995), ‘Zero-AOX shrinkproofing treatment for wool top and fabric. Part 1: glow discharge treatment’, Text Res J, 65, 355–361. 31 THOMAS H(2007), ‘Plasma modification of wool’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. 228–246. 32 CHAIVAN P, PASAJA N, BOONYAWAN D, SUANPOOT P AND VALAITHONG T (2005), ‘Low-temperature plasma treatment for hydrophobicity improvement of silk’, Surface Coat Technol, 193, 356–360. 33 SELLI E, RICCARDI C, MASSAFRA M R AND MARCANDALLI B (2001), ‘Surface modifications of silk by cold SF6 plasma treatment’, Macromol Chem Phys, 202, 1672–1678. 34 JOHANSSON K (2007), ‘Plasma modification of natural cellulosic fibres’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. 247–281. 35 SELLI E, MAZZONE G, OLIVA C, MARTINI F, RICCARDI C, BARNI R, MARCANDELLI B AND MASSAFRA M R (2001), ‘Characterisation of poly(ethylene terephthalate) and cotton fibres after cold SF6 plasma’, J Mater Chem, 11, 1985–1991. 36 CAI Z, QIU Y, ZHANG C, HWANG Y-J AND MCCORD M (2003), ‘Effect of atmospheric plasma treatment on desizing of PVA on cotton’, Text Res J, 73, 670–674. 37 KHAMMATOVA V V (2005), ‘Effect of high-frequency capacitive discharge plasma on the structure and properties of flax and lavsan materials’, Fibre Chem, 37, 293–296. V 38 VRABI C, JESIH A AND SVETEC D G (2007), ‘Physical and absorptive changes in plasma treated viscose fibres’, Fibres Text, 15, 124–126. 39 CAI Z, QIU Y, HWANG Y-J, ZHANG C AND MCCORD M (2003), ‘The use of atmospheric plasma pressure treatment in desizing PVA on viscose fabrics’, J Ind Text, 32, 223–232. 40 DENES F, YOUNG R A AND SARMADI M (1997), ‘Surface functionalization of polymers under cold plasma conditions – a mechanistic approach’, J Photopolym Sci Technol, 10, 91–112. 41 LEE S D, SARMADI M, DENES F AND SHOHET J L (1997), ‘Surface modification of polypropylene under argon and oxygen-RF-plasma conditions’, Plasmas and Polymers, 2, 177–198. 42 SIGURDSSON S AND SHISHOO R (1997), ‘Surface properties of polymers treated with tetrafluoromethane plasma’, J Appl Polym Sci, 66, 1591–1601. 43 RICCARDI C, BARNI R, SELLI E, MAZZONE G, MASSAFRA M R, MARCANDALLI B AND POLETTI G (2003), ‘Surface modification of poly(ethylene terephthalate) fibers induced by radio frequency air plasma treatment’, Appl Surf Sci, 211, 386–397. 44 POLETTI G, ORSINI F, RAFFAELE-ADDAMO A, RICCARDI C AND SELLI E (2003), ‘Cold plasma treatment of PET fabrics: AFM surface morphology characterisation’, Appl Surf Sci, 219, 311–316. 45 NEGULESCU I I, DESPA D, CHEN J, COLLIER B J, DESPA M, DENES A, SARMADI M AND DENES F S (2000), ‘Characterizing polyester fabrics treated in electrical discharges of radio-frequency plasma’, Text Res J, 70, 1–7.

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46 YIP J, CHAN K, SIN K M AND LAU K S (2002), ‘Low temperature plasma-treated nylon fabrics’, J Mater Process Technol, 123, 5–12. 47 WEI Q F, GAO W D, HOU D Y AND WANG X Q (2005), ‘Surface modification of polymer nanofibres by plasma treatment’, Appl Surf Sci, 245, 16–20. 48 BHAT N V AND BENJAMIN Y N (1999), ‘Surface resistivity behavior of plasma treated and plasma grafted cotton and polyester fabrics’, Text Res J, 69, 38–42. 49 REN C S, WANG D Z AND WANG Y N (2006), ‘Graft co-polymerization of acrylic acid onto the linen surface induced by DBD in air’, Surf Coat Technol, 201, 2867–2870. 50 ROSACE G AND MASSAFRA M R (2008), ‘Marking of cellulose yarn by vinyl monomer grafting’, Text Res J, 78, 28–36. 51 CHOI H-S, KIM Y-S, ZHANG Y, TANG S, MYUNG S-W AND SHIN B-C (2004), ‘Plasmainduced graft co-polymerization of acrylic acid onto the polyurethane surface’, Surf Coat Technol, 182, 55–64. 52 MALEK R M A AND HOLME I (2003), ‘The effect of plasma treatment on some properties of cotton’, Iranian Polym J, 12, 271–280. 53 YUEN C W M AND KAN C W (2007), ‘Influence of low-temperature plasma on the inkjet-printed cotton fabric’, J Appl Polym Sci, 104, 3214–3219. 54 OKUNO T, YASUDA T AND YASUDA H (1992), ‘Effect of crystallinity of PET and nylon 66 fibers on plasma etching and dyeability characteristics’, Text Res J, 62, 474–480. 55 RAFFAELE-ADDAMO A, SELLI E, BARNI R, RICCARDI C, ORSINI F, POLETTI G, MEDA L, MASSAFRA M R AND MARCANDELLI B (2006), ‘Cold plasma-induced modification of the dyeing properties of poly(ethylene terephthalate) fibers’, Appl Surf Sci, 252, 2265– 2275. 56 RIGBY A J, ANAND S C AND HORROCKS A R (1997), ‘Textile materials for medical and healthcare applications’, J Text Inst, Part 3, 88, 83–93. 57 HUH M W, KANG I-K, LEE D H, KIM W S, LEE D H, PARK L S, MIN K E AND SEO K H (2001), ‘Surface characterization and antibacterial activity of chitosan-grafted poly(ethylene terephthalate) prepared by plasma glow discharge’, J Appl Polym Sci, 81, 2769–2778. 58 TYAN Y-C, LIAO J-D AND LIN S-P (2003), ‘Surface properties and in vitro analyses of immobilized chitosan onto polypropylene non-woven fabric surface using antennacoupling microwave plasma’, J Mater Sci Mater Med, 14, 775–781. 59 COULSON S (2007), ‘Plasma treatment of textiles for oil and water repellency’, in Plasma Technologies for Textiles, Shishoo R (Ed.), Woodhead Publishing Ltd, Cambridge, pp. 183–201. 60 MATHER R R, ROBSON D, FOTHERINGHAM A F, NEVILLE A, WEI Q AND WARREN J M (2003), ‘Effects of gas plasma treatments of textiles on their technological and aesthetic properties’, in Proceedings of the International Textile Design and Engineering Conference (INTEDEC), Heriot-Watt University, Edinburgh, September, Section 7B.

14 Emerging approaches to the surface modification of textiles Q. WEI

Jiangnan University, China

Abstract: New technologies have been increasingly adopted for the modification of textile materials in order to achieve new functions. This chapter reviews some new approaches to the surface modification of textile materials. These new techniques have great potential in the textile industry and will give rise to new functions that could not be achieved using previously available techniques. Future developments in the surface modification of textile materials are also discussed in this chapter. Key words: surface modification, atom transfer radical polymerization, molecular imprinting, biomimetic.

14.1

The expansion of textiles into technical applications

Textile materials, with their unique performance characteristics, have been widely used in various industrial areas. They are frequently used as civil engineering textiles, aquaculture and marine materials, medical treatment materials, filtration materials, agriculture and forestry textiles, textiles for various types of vehicles, and many other applications. Textile materials represent a new fourth type of civil construction material following steel, cement and timber, and they play an important part in the reinforcement, drainage, filtration, isolation (including anti-seeping functions) and protection of soils. These excellent characteristics of textile materials have aroused great interest and attention in the engineering sector. They are used in order to extend the life of materials for civil engineering, shorten construction time, save raw materials, lower construction costs and simplify maintenance. Agricultural and horticultural textiles represent a new category of industrial textiles. People use them in water and soil conservation, vegetable production, soil-less cultivation and drainage. In addition, they can also be used in the flower industry, for vegetable packaging and for storage. Textile materials also play an important part in the medical industry. With the research and development of textile materials and continuous innovations in biomedical products, medical textiles are increasing rapidly. From medical sutures, 318

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319

gauzes and bandages to repair fabrics, and from functional materials to a variety of artificial prostheses, medical textiles represent a large proportion of the textile materials market and are becoming increasingly high-tech for the construction of integrated products. Ocean fishing, aquaculture, marine mineral resources extraction and the development and utilization of energy all make use of textile fibers, especially synthetic fibers. Synthetic fibers with high mechanical performance, low hypobarism, good weather resistance, good corrosion resistance and other characteristics are widely used in the manufacturing of artificial reefs and extraction of seaweed and algae. Nylon, polyethylene terephthalate (PET) and other synthetic fibers with high mechanical performance are much better than natural fibers for marine development. Transportation textiles can be used in various types of vehicles, such as cars, aerospace vehicles, trains and ships. These fabrics, from simple decorative materials in cars to composite functional materials, possess high strength, high toughness, resistance to ultraviolet light and wear-and-tear, and other major performance characteristics that are important to the vehicles’ low cost and high safety requirements. Moreover, textile materials are also widely used in the military and national defence industries, in the sports and leisure industries, and many other areas. In very many of these applications, the technology relating to the surface function of textiles is also developing at an equivalent rate.

14.2

New techniques for surface modification

In addition to the various technologies discussed above, there are still several new types of functionalisation technologies that need to be introduced: atom transfer radical polymerization (ATRP), ultrasonic waves, molecular imprinting, ionic liquids and biomimetic approaches.

14.2.1 Atom transfer radical polymerization ATRP is a recently developed method for controlling radical polymerization (Zhou et al., 2008), and has been one of the most widely employed techniques, involving a fast dynamic equilibrium between dormant species and active radical species to provide control of the polymerization (Hou et al., 2006). It can achieve better control of polymer molecular weights, molecular weight distribution, and tolerances to various reaction conditions and can be applied to various monomers (Mittal et al., 2007). Therefore, ATRP is suitable for use in the preparation of functional polymers, having excellent properties, from much more complex macromolecular systems (Xu et al., 2008). Metal-mediated ATRP is a versatile technique for the synthesis of a wide range of materials derived from all kinds of monomers such as styrenes, methacrylates and acrylates (Zhang et al., 2008).

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Aqueous ATRP has also received attention recently because of its ability to synthesize polymers at room temperature (Mittal et al., 2007).

14.2.2 Ultrasonic waves Ultrasonic waves are acoustic waves with frequencies between 10 and 20 MHz. When an ultrasonic wave is propagated in various media, several concomitant physical effects – such as mechanical, thermotic and cavatition effects – can occur. These effects are recognized as being beneficial to many physical and chemical processes (Liu et al., 2007). For example, heat transfer processes, chemical reactions, fine particle preparation and waste-water treatment all benefit from ultrasonic irradiation (Liu et al., 2007). Hurren and coworkers, for example, have shown that, compared with hand washing, ultrasonic agitation has many advantages. It has negligible effects on the strength and color of fabrics and causes less fiber migration. Furthermore, it can increase the level of stain removal from the fabric and reduce felting and area shrinkage during the laundering of wool fabrics (Hurren et al., 2008). In fact, with the demand for improvements in the integrity of materials, ultrasonics have been used under various conditions. Non-destructive testing by ultrasonic waves has great importance in investigating the mechanical behavior of materials. Ultrasonic waves are often emitted and received by transducers using a piezoelectric ceramic as the sensitive element (Si-Chaib et al., 2000). Ultrasonic extraction (UE), described by Rezic (2008), can be used for both liquid and solid samples, and for the extraction of either inorganic or organic compounds. Ultrasonic diffusion is considered to be one of the most useful processes for diffusing molten aluminum into carbon fiber bundles (Tadashi et al., 2007). The improved ultrasonic atomizer method has been used to deposit sensing films on quartz crystal microbalance (QCM) sensors (Wyszynski et al., 2007).

14.2.3 Molecular imprinting Molecular imprinting is a method of inducing molecular recognition properties in synthetic polymers in response to the presence of a template species during the formation of the three-dimensional structure of a polymer (Mayes and Whitcombe, 2005). The history of molecular imprinting can be traced back to the experiments of Dickey conducted in the 1940s and 1950s. The limitations of traditional applications of molecular imprinting polymers (MIPs) are long preparation times, mechanical deformation of the binding sites during grinding of bulk polymers, and the time-consuming sieving procedure necessary for isolation of the fraction with a narrow size distribution that is associated with high material losses (Sergeyeva et al., 2007). New MIP formats are being developed that avoid these limitations; highly porous self-supported MIP membranes provide a typical example.

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14.2.4 Ionic liquids Recently, increased attention has been focused on ionic liquids (Patsahan, 2005). Room-temperature ionic liquids, which can meet all of the requirements for ideal electrolytes – including high ionic conductivity, large electrochemical windows, excellent thermal and electrolyte stability, and negligible evaporation (Lu and Mattes, 2005) – have been introduced to obtain conducting polymers with high actuation performance over an extended period of time. However, although roomtemperature ionic liquids have potential importance in supercapacitors and fuel cells, it is difficult to make full use of them. The toxicity of ionic liquids has been found to be similar to or greater than that of many of the current solvents already in use (Pretti et al., 2006); further research is necessary to solve these problems.

14.2.5 Biomimetic approaches Researchers are paying more and more attention to these special materials, such as polypeptides in animals, polysaccharides in plants and chitin in insects, arthropods and crustaceans. By making full use of them, biology will bring us a vast array of materials (Jeronimidis, 2007). The biomimetic approach can be used to study the adsorption of a model copolymer and for representing the lignin–carbohydrate complex (Gradwell et al., 2004). It is interesting to speculate about employing biology to produce new types of fibers and even functional materials at room temperature, as it would clearly make the whole world very different.

14.3

Future trends

Science and technology have undergone profound changes in recent years, with the development of micro-electronics technology, organic polymer materials and biological engineering technology. A large number of high-tech industries have been formed, and the scientific community considers high-performance, multifunctional fiber materials as the direction of technological progress in fibers. In turn, their enhanced performance is assisting development and progress in aviation, aerospace, high-speed transport, marine engineering, new construction, new forms of energy delivery, environmental industries, national defense and cutting-edge fields of science. Over recent years, with the continuous development of the technology and the increasing demand for high-tech industrial textiles, these new fibers are developing quickly. In Europe, the United States and other developed countries, industrial textiles constitute 30% of all textiles manufactured (Technical Textiles and Nonwovens, 2007). As science and technology advances, textile materials are also continuously expanding their applications into the fields of environmental protection, biomedical applications, new construction areas, home appliances, energy industries, nuclear reactor waste-water treatment areas and many other fields. All these fields

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demand textile materials to be sustainable, multi-functional and green, and to be obtained using biotechnology, as people pay more and more attention to their living environment.

14.4

References

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ultrasonic infiltration method’, Composites: Part A, 38, 771–778, doi: 10.1016/ j.compositesa.2006.09.003. TECHNICAL TEXTILES AND NONWOVENS (2007), ‘Technical textiles and nonwovens’, Coating, 40(5), 2–8. WYSZYNSKI B, GUTIERREZ-GALVEZ A AND NAKAMOTO T (2007), ‘Improvement of ultrasonic atomizer method for deposition of gas-sensing film on QCM’, Sensors and Actuators B, 127, 253–259, doi: 10.1016/j.snb.2007.07.052. XU Q, LI N, YAN F, XIA X, LU J AND WEN X (2008), ‘Atom transfer radical polymerization of a novel quinoline derivative monomer and fluorescent property’, European Polymer Journal, 44, 1874–1880, doi: 10.1016/j.eurpolymj.2008.01.004. ZHANG L, CHENG Z AND SHI S (2008), ‘AGET ATRP of methyl methacrylate catalyzed by FeCl/3 iminodiacetic acid in the presence of air’, Polymer, 49, 3054–3059, doi: 10.1016/ j.polymer.2008.04.057. ZHOU Y, WANG S, DING B AND YANG Z (2008), ‘Modifcation of magnetite nanoparticles via surface-initiated atom transfer radical polymerization (ATRP)’, Chemical Engineering Journal, 138, 578–585, doi: 10.1016/j.cej.2007.07.030.

Index

AATCC Test Method 61-2006, 173 AATCC Test Method 100-2004, 118 AATCC Test Method 124-2006, 118 AATCC Test Method 183-2006, 173 AATCC TM 82, 274 absorbency, 118 acrylic acid, 96, 103, 312 acrylnitrile monomer, 92 adhesion force, 82–5 abrasion test, 84 abrasion resistance, 85 sample details, 85 coating adhesion, 85 indentation adhesion, 85 peel-off test, 82–4 adsorbable organic halide, 312 adsorbable organohalogens, 291 aerogels, 137 Aerosil particles, 200 Aeroxide P25, 208 agglomeration, 165, 166 Agilent-E8363A, 119 air plasmas, 311 air–helium plasma treatments, 307 air-jet texturing system, 249 air–oxygen–helium plasma treatments, 307 A-layer, 305 alkaline pectate lyases, 146, 147 alkoxysilanes, 206 alkylsulfate salts, 286 alkylsulfonate salts, 286 allylamine plasma deposition, 252 aluminium oxide films, 136 aluminium-doped zinc oxide coating, 75 ammonia, 282 ammonia plasma treatment, 251 ammonium peroxydisulfate, 92 amylase, 292 anatase, 176, 207, 208

anionic softeners, 287 anti-bacterial properties, 121–2 anti-static property, 119 aqueous solutions mechanisms and chemistries of textile surface modifications, 270–9 for surface modification of textiles, 269–93 aramid, 245, 253–5, 261 aramides, 196 argon, 299 argon plasma, 306, 307 Armos aramid fibre, 254 Arrhenius dependence on temperature, 131 ascorbic acid, 151 Aspergillus niger, 143 ASTM D 3691-2002, 118 ATCC 6538, 121 ATCC 11229, 121 atmospheric plasma, 99 atmospheric-pressure glow discharge, 299 atmospheric-pressure plasmas, 298–9 atom transfer radical polymerisation, 319–20 atomic force microscopy, 165, 225, 249 force analysis, 43–4 phase analysis, 42 PLLA-PVA composite nanofibres, 43 schematic presentation, 40 surface and interface analysis, 41–2 nano-coated layer and mica surface, 42 PET fibre surface morphology, 41 atomic layer deposition, 135–6 β-α inverting mechanism, 143

325

326

Index

Bacillus subtilis, 143 balance twist, 244 β-β retaining mechanism, 143 1,4-β-cellobiosidase, 143 bending rigidity, 116 mechanical performance of untreated, copper-plated and nickel-plated polyester fabrics, 117 bending tests, 197 benzyl methacrylate, 103 Berger criterion, 194 β-glucosidase, 143 biomimetic approaches, 321 biopolishing, 143–4 biotechnology, 140–1, 292–3 bleaching, 6–7, 269 bromine-etched Kevlar fibre, 255 Brownian motion, 258 Brunauer–Emmett–Teller surface area, 223 Burlington Industries, 181 butadienenitrogen, 250 butanetetracarboxylic acid, 278 cadmium sulfide, 93 caffeic acid, 151 calendering, 261 carbon tetrachloride, 255 carbonyl, 306 carboxylic groups, 306 carboxymethylation, 278 Carroll’s approach, 52 CASING technique, 308, 314 cationic softeners, 287 caustic soda, 271, 282 cellulase, 142–4, 292 cellulose, 94–5 binding domains, 156 chemo-enzymatic functionalisation, 149 fibres, 271–2, 273–4 hydrolysis, 143 via β-β retaining or β-α inverting mechanism, 145 polymer, reducing and non-reducing ends, 144 centrifugal draw spinning, 261 ceramic matrix composites, 241 chemical grafting, 92–3

graft yield as a function of grafting solution temperature and reaction time, 94 graft yield as a function of monomer concentration and reaction time, 95 chemical interactions, 239–40 chemical vapour deposition, 216 applications, 133–6 coating from a vapour over profiled substrate, 133 coatings characteristics, 132–3 current trends and potential advances in uses and techniques, 136–7 nanocrystalline silicon on top of aluminium conducting layer, 128 practical methods, 129–32 processes occurring during CVD on textile, 129 reactor design, 131 forms, 131–2 textile surface functionalisation, 126–37 chitosan, 19, 149–50, 312 Chloramine Fast Red K, 311 chlorination, 289 chlorine/Hercosett process, 291, 312 chlorogenic acid, 150, 151 chlorosulfonil, 255 chondroitin sulphate, 222 chromic acid oxidation, 251 Chromobacterium violaceum, 182 cold-wall reactor, 131 collagen, 222 colloidal activation, 110 colloids, 167 composite coatings, 78–9 composites aramid fibre structure, 253 basics, 239–40 adhesion, 239–40 contact angle measurement, 241 surface energy, 240 classification ceramic matrix composites, 241 metal matrix composites, 241 polymer matrix composites, 241 cord layers in tractor tire, 245 future trends, 262–4

Index PET structure, 247 polyethylene naphthdate cord, 244 structure, 241–2 surface modification of textiles, 239–45 surface properties of reinforcing fibres and applications, 246–57 carbon fibre, 255–7 high-modulus polyethylene fibres, 250–5 polyesters, 246–50 tire cord technology, 243–5 cord–rubber adhesion, 244–5 key tire components, 243 structure of a tire, 243 tire cord construction, 244 conductivity, 119 contact angle, 13, 50–3 barrel-shaped droplets on single fibre, 52 and interfacial tensions, 50 oxygen-plasma-treated and untreated samples, 14 sessile drop method, 51–2 droplet shape on PTFE surface, 51 Wilhelmy technique, 53 contact angle measurement, 240, 249 continuous draw spinning, 261 continuous roll-to-roll systems, 299 CoolMax fibre, 283 copper coatings, 64–7 dynamic contact angles of PET fibre, 67 surface roughness of PET fibre, 66 corn fibre, 273 corona discharge, 9, 298 Corynebacterium sp, 151 cotton, 142–4, 144, 221, 306–7 mercerisation, 280–2 morphological changes during mercerisation, 281 stain-repellent finishes, 284–6 Coulomb repulsion, 167 cuprammonium hydroxide, 274 cupriethylene diamine hydroxide, 274 cuticle, 304 cutinase, 147, 151–5 CVD see chemical vapour deposition cyclodextrins, 19

327

degumming, 305 denier reduction, 289 depilling, 143–4 desizing, 6 diammonium hydrogen phosphate, 196 diborane, 130, 136 dichlorodicyanuric acid, 289 dielectric barrier discharge, 8, 298–9 diethylene glycol terephthalate, 283 diethylene triamine, 252 differential scanning calorimetry, 92, 153 dimethylol dihydroxyl ethylene urea, 277 dip-coated film gel, 215 4,4-diphenylmethane diisocyanate, 278 dip-pad–dry-cure process, 176 direct oxygen plasma, 251 directional frictional effect, 304 disulfide bond, 275 dual-action fluoro-copolymers, 286 Dupont, 254 dyeing assay, 153 electroless deposition modification of textile, 108–23 copper- and nickel-plated polyester fabrics, 112, 115–22 future trends, 123 strengths and weaknesses, 122–3 techniques and key principles, 109–12 electroless plating activation, 111 composition and condition of copper and nickel plating, 111–12 evaluation, 112 post-treatment, 112 pre-cleaning, 111 sensitisation, 111 electromagnetic interference shielding, 119–20 untreated, copper-plated & nickelplated polyester fabric, 120 electron spectroscopy for chemical analysis/X-ray electron spectroscopy, 153 electrostatic attraction theory, 239 electrostatic stabilisation, 166 EMI shielding effectiveness, 119 encapsulation, 180

328

Index

endo-β-1,4-glucanase, 143 endocuticle, 304 endoglucanases, 143 endopolygalacturonase lyases, 146 endopolygalacturonases, 146 endopolymethylgalacturonate lyases, 146 energy dispersive X-ray analysis, 48–50 alginate fibre before and after ion exchange, 49 environmental scanning electron microscope, 35–9 with cryo-stage, 39 dynamic characterisation, 37–9 water absorption of alginate fibre, 38 schematic presentation, 36 surface and interface characterisation biodegradation of PLLA fibre, 38 wetting behaviours of meltblown PP fibre, 37 environmental scanning electron microscopy, 154 enzymes surface modification, 139–57 cellulose hydrolysis, 143 cellulose polymer, reducing and nonreducing ends, 144 cellulose polymer hydrolysis via β–β retaining or β–α inverting mechanism, 145 chemo-enzymatic functionalisation of cellulose, 149 enzymes, technologies and materials, 142–55 cellulases for cotton, Tencel, viscose and linen, 142–4 cutinase for PET, 151–5 pectinases in enzymatic scouring of cotton, 144, 145–7 tyrosinase for chitosan and protein fibres, 149–51 XET for cellulosic materials, 147–9 and functionalisation of textiles, 140 future trends, 156–7 hydrolysis of PET and polyamide, 153 inkjet technology, 154 principles, 139–42 reaction of o-quinone with amino group, 150 reaction schema of tyrosinase, formation of o-quinone, 150

strengths and weaknesses, 155–6 epichlorohydrin-grafted Kevlar fibre, 255 epicuticle, 304 e-polytetra fluoroethylene, 261 epoxy chloropropane, 254 Escherichia coli, 169 esterase, 293 ether, 306 Evonik, 203 exopolygalacturonase lyases, 146 exopolygalacturonases, 146 F-12 aramid fibre, 254 fabric appearance, 118–19 fabric shrinkage, 116–17 fabric softeners, 286–8 Faraday’s method, 167 ferric chloride solution, 134 fibre surface reduction, 275 fibroin, 305 filtration applications of surface-modified fibres, 260–2 filtering media hierarchy, 259 future trends, 262–4 mechanisms, 257–8 diffusion, 258 direct interception, 258 electrostatic attraction, 258 gravitational deposition, 258 inertial impaction, 258 principles, 257–60 surface modification of textiles, 257–60 flame attenuation spinning, 261 flame retardation, 99, 194 flame treatment, 9 flow-through process, 131 fluorescein isothiocyanate, 148 fluorinated alkoxysilane, 198 fluorinated polymer coatings, 135 fluoroalkylsilane, 225 fluorocarbon, 174, 299, 308, 313 fluorocarbon plasma, 305, 309 fluoro-chemical finishes, 285–6 1-fluoro-2,4-dinitrobenzene, 278 fluoromethylic copolymer coating, 177 fluoropolymer, 308 fluoropolymer coating, 313

Index fluoro-polymer-based materials, 277 formaldehyde, 111 Fourier transform infrared spectroscopy, 44–6, 151, 153, 225, 229, 302 PAN nanofibres, ZnO-coated PAN nanofibres, pre-oxidised and carbonised ZnO-coated PAN nanofibres, 45 Fourier transform-Raman spectroscopy, 151 fuming nitric acid treatment, 251 Fusarium oxysporum, 152 Fusarium solani pisi, 147, 152, 153 gelatine, 166 germane, 136 Gibbs free energy, 166 glass fibre, 200, 260, 261–2 glass transition temperature, 276 glycidyl methacrylate, 100 grafting, 270, 310 graphite, 131 Grashof number, 132 Green-shield, 178 hand builders, 286–8 helium, 299, 310 helium–oxygen plasmas, 310 hemp fibre, 18 Scourzyme-treated, 18 high-performance liquid chromatography, 153 Hi-Nicalon silicon carbide fibre, 134 hot-wall reactor, 131 H-pull test method, 249 H–T, 248 Humicola insolens, 143 hydrogen peroxide, 275 hydrolysed poly[2-(3-thienyl) ethanol butoxy carbonyl-methyl urethane], 223 hydrophilic fibres, 271 hydrophobins, 140, 157 indium tin oxide, 227 indium-doped tin oxide coating, 72–5 optical properties of PET non-woven material, 74 resistivity of PET non-woven material, 74

329

initiated CVD, 135 ink-jet printing, 311 inkjet technology, 154 interdiffusion theory, 239 interfacial bonding, 79–80, 82–5 adhesion force, 82–5 abrasion test, 84 coating adhesion, 85 indentation adhesion, 85 peel-off test, 82–4 mechanism of adhesion, 79–80 microstructures of interfaces, 80, 82 coated fibres, plasma pre-treatment, heat treatment, 81 ITO-coated fibres, 80 interfacial shear strength, 252 inter-fibre bonding, 291 interlaminar shear strength, 251 ion implantation, 60 ionic liquids, 321 ionised gas treatments, 8–9 corona discharge, 9 flame treatment, 9 plasma treatment, 8–9 ultraviolet irradiation, 9 isothermal titration calorimetry, 220 ITV Denkendorf, 177 Keggin structure, 229 Kevlar, 253 Klebsiella pneumoniae, 171, 172 Köstrosol, 194 K/S value, 222 laccase, 157, 221 Langmuir–Blodgett films, 215 Laplace’s law, 52 laundry tests, 197 layer-by-layer deposition methods CA fibrous membranes deposited with various bilayer of TiO2/PAA SEM images, 227 water contact angle and water-roll angles, 228 conclusions and future trends, 232 cross-sectional fibres calcined at 380 ºC, 230 layer-by-layer assembly, 216–18 via electrostatic interactions, 217

330

Index

LbL deposition on textile surfaces, 221–31 deposition on electrospun nanofibres, 223, 225, 227 deposition on traditional textile fibres, 221–2 nanofibre production process, 223 nanofibres as template for nanotubes or hollow nanofibres, 229–31 strengths and weaknesses, 231 LbL-coated fibrous mats, 224 for nano-modification of textile surfaces, 214–32 POM nanotubes fabrication, 229 super-hydrophobic surfaces preparation, 226 technique, 218–20 experimental considerations for textile materials, 220 key principles, 219 mechanism, 219–20 methods, 218–19 uncoated and film-coated fibrous mats, 225 LbL see layer-by-layer deposition methods Levasil, 194 light induced grafting, 100–1 see also ultraviolet radiation-induced surface grafting linen, 142–4, 307 lipase, 292 liquid phase deposition process, 215 Lotus-Effect, 174, 175, 180, 200, 207, 292 low-pressure plasmas, 297–8 lumen, 306 Lyocell, 142 lyogels, 186 magnetron sputtering, 63–75, 77–9 metallic coatings and their properties, 63–8 copper coatings, 64–7 other coatings, 68 silver coatings, 63–4 oxide coatings and their properties, 68– 75, 77 aluminium-doped zinc oxide coating, 75, 77

indium-doped tin oxide coating, 72–5 titanium dioxide coating, 68–70 zinc dioxide coating, 70–1 maleic acid anhydride, 257 Martindale test, 197 mass-transfer limited, 131 mechanical interlocking, 239 mercerisation, 7 merino wool fibre, 14 untreated, N2 post-discharge, O2 postdischarge treatment, 17 metal matrix composites, 241 metallisation, 108–9 methacrylamide, 93 methacryloyl chloride, 255 methyl ethyl ketoxime, 278 methylsilane, 136 micro-electro-mechanical systems, 216 microwave plasma, 176 molecular imprinting polymers, 320 Moore’s law, 165 multi-walled carbon nanotube, 231 N–3, 248 NANO-CARE, 180, 181 nanocrystals, 167 NANO-DRY, 181 Nano-Dry finishing, 284 NANO-FRESH, 181 NanoMATRIX, 181 Nanomer, 187 nanoparticles antibacterial finishing of cotton fabrics with nano-ZnO, 172 characterisation techniques, 170 commercialisation of nanofinishing in textiles, 180–1 completely dispersed silver nanoparticles, 167 cotton fibre impregnated with silver nanoparticles, 171 design and electricity-generating mechanism of the fibre-based nanogenerator, 178–9 electrostatic stabilisation of metal colloids, 168 functional properties, 169–80 antimicrobial finishes, 169–71

Index easy-care finishes, 174–7 other functional finishes, 178, 180 ultraviolet-protection finishes, 171–4 lotus leaf, 175 Lotus-Effect removing dirt particles from super-hydrophobic surfaces, 175 nano-ZnO coated cotton fibres, 173 photocatalytic behaviour of titania nanoparticles, 177 super-hydrophobic textile material created using nanotechnology, 176 synthesis and characterisation, 165–9 for textile surface modification, 164–82 future trends, 182 strengths and weaknesses, 182 top-down and bottom-up approaches for synthesis of nanomaterials, 166 UV-transmittance characteristics of textile material, 172 Nano-PEL, 181 nanorobots, 165 nanosols, 186 NanoSphere technology, 180 nanotechnology, 19–20, 136, 181, 214, 263, 292 definition, 164 Nano-Tex, 180, 181 NANO-TOUCH, 181 nanowhiskers, 180, 181 natural fibres animal origin, 3 important animal fibres, 5 plant origin, 2–3 important plant fibres, 4 Nelumbo nucifera, 174 N-halamine siloxanes, 273 nickel carbonyl, 127 nitration method, 274 nitrogen, 299 nitrogen plasmas, 311 Nomex, 253, 260 non-ionic softeners, 287 nonwovens, 11 SEM images of untreated polypropylene nonwovens and after plasma treatment, 12 Novozymes, 146 nucleation, 109–11

331

Nyacol Nano-technologies Inc., 178 nylon, 261 nylon 6, 227, 249, 310 nylon 66 cords, 249 olyester, 261 optical microscopy, 197 organically modified ceramics, 187 Organosil, 187 Ormocer, 187 orthophosphoric acid, 196 Ostwald ripening, 165, 166 oxygen plasma, 251, 307, 309 ozone bleaching, 14–16 C1, O1 and N1 relative intensities, 17 pad–dry–cure method, 270, 283, 285, 288, 291 PA6-organically modified montmorillonite, 75, 77 differential scanning calorimetry analysis, 76 modified by Fe2O3 magnetron sputter coating, 76 paraffin, 276, 277 Peclet number, 132 pectin esterase, 144, 146 pectin lyases, 145 pectinases, 144 peel tests, 249 pentaerythritol, 252 perfluorooctyl-2 ethanol acrylic monomer, 93 perfluorooctyl sulfonate, 286 peroxidase, 292 PET see poly(ethylene terephthalate) Pexul, 248 phosphate esters, 196 phosphine, 130, 136 photocatalytic behaviour, 207 photo-CVD, 129 physical vapour deposition, 216 textile surface functionalisation, 58–86 future trends, 86 interfacial bonding, 79–85 sputtering, 63–75, 77–9 working principles, 59–62 piezoelectric zinc oxide nanowires, 180 plasma treatment, 8–9, 216

332

Index

applications, 311–13 biomedical textiles, 312 oil and water repellency, 313 textile dyeing, 311 textiles in biomedical applications, 313 wool shrinkproofing, 311–12 characterisation of plasma-treated textile surfaces, 301–3 chemical techniques, 302 other approaches, 303 physical and topographical techniques, 301–2 effects of some gas plasmas on textile surfaces, 304 future trends, 313–14 low-pressure vs atmospheric-pressure treatments, 299–300 modifications to textiles surfaces, 303–10 cellulosic textiles, 306–7 grafting, 310 protein textiles, 304–6 synthetic textiles, 307–10 nature of plasmas, 297 non-thermal, 297 thermal, 297 plasma generation, 297–9 atmospheric-pressure plasmas, 298–9 low-pressure plasmas, 297–8 power supply low frequency, 297 microwave, 297 radiofrequency frequency, 297 strengths and limitations, 300 for surface modification of textiles, 296–314 plasma-enhanced CVD, 127, 129 plasma-induced grafting, 96–7, 99–100 FTIR spectra of vinyl laurate monomer grafted on the cotton fabric, 98 variation of the amount of grafted monomer as a function of the monomer concentration, 98 plasmons, 169 poly(acrylic acid), 218, 225 polyacrylonitrile, 100, 196, 255 poly(allylamine), 218, 221, 225

polyamide, 153, 196, 200 polycaprolactone, 135 poly(diallyldimethylammonium chloride), 218, 219 polydimethylsiloxane, 276, 277 polyelectrolyte multilayers, 221, 222, 225, 227 polyester, 137, 196, 200, 245, 246–50, 271, 278, 309 surface hydrolysis, 288–9 polyester fabric carbon (1s) XPS spectra, 13 copper- and nickel-plated, 112, 115–22 changes in weight and thickness after electroless plating, 116 functional properties, 119–22 mechanical performance, 115–19 microstructure, 112, 115 scanning electron micrographs, 113 X-ray diffraction, 114 laser modification, 10 polyester fibre, 11 polyethylene, 137, 276 polyethylene oxide, 283 poly(ethylene terephthalate), 20, 22, 151– 5, 153, 222, 249, 271, 309, 312 oxygen plasma treatment, 21 surface topography, 21 poly(ethylene terephthalate) fabrics, 101 poly(ethyleneimine), 166, 218, 225, 227 polygalacturonases, 144, 146 poly(L-lactic acid), 273 polymer coatings, 77–9 dynamic contact angle of silk fabric, 78 polymer matrix composites, 241 polymeric quaternary ammonium, 135 polymerised siloxanes, 285 poly(methacrylic acid), 222 polymethylsiloxane coating, 134 polyol route, 168 polyolefines, 196 polyoxometalate, 225, 227 poly-perfluoroalkyl ethyl methacrylate, 135 poly(phenylene terephthalamide), 255 poly(phthalazinone ether sulfone ketone), 256 poly(p-phenylene-2,6-benzobisoxazole), 250

Index polypropylene, 260, 261, 278, 308, 312 polypropylene fibre, 13–14 untreated and with oxygen plasma treatment, 15 XRD results of untreated and with oxygen plasma treatment samples, 16 poly(propylene glycol), 278 polypyrrole, 133–4, 137 poly(styrenesulfonate), 218, 221 polytetrafluoroethylene, 196 poly(trimethylene terephthalate), 273 poly(vinyl alcohol), 166, 227, 307 poly(vinyl pyrrolidone), 166 poly(vinylsulfate), 218 Populus tremula x tremuloides, 148 protease subtilisine, 157 proteases, 292 protein fibres, 149–51 proteins, 166 Pseudomonas mandocino, 147 Pseudomonas putida, 151 pulsed XeCl, 252 pyrophoric silane, 136 quantum dots, 167 quartz crystal microbalance, 221, 320 quasi molecular, 206 quaternary ammonium salts, 276 quaternised chitosan, 312 radiation-induced grafting, 94–6 radiofrequency plasma, 97, 99, 176 Raman spectra, 231 rare earth solution, 254 reaction chamber pressure, 130 repellent power, 177 resorcinol–formaldehyde–latex adhesive system, 245 roll-to-roll process, 131 Sachtleben, 203 scale masking, 291 scanning electron microscopy, 27–32, 169, 177, 197, 225, 229, 231, 249, 273, 301 compositional contrast, 32 crystallographic phases, 32 interface characterisation, 31–2

333

sputter-coated layer and fibre interfacial bonding, 31 schematic representation, 28 structural characterisation, 28–30 structured nanofibres, 29 surface characterisation, 30–1 surface evolution of PAN nanofibres, 30 scanning probe microscopy, 39–44 atomic force microscopy schematic presentation, 40 scanning tunneling microscopy, 39 schematic presentation, 40 scanning tunnelling microscopy, 165 Scarlet dye, 222 scouring, 6, 306 Scourzyme L, 18 sericin gum, 305 sericin peptides, 150 silanols, 186 silica sol, 185 silicic acids, 186 silicon oxide coatings, 132 silicone softeners, 287 silicones, 276 silk, 305–6 siloxane, 276 silver, 169 silver coatings, 63–4 effect on EMI shielding efficiency, 65 original PP fiber and with silver film, 64 silver nitrate, 168 simple padding process, 202, 203, 204 singeing, 6, 261 single-dip system, 248 sintering, 165 Snowtex, 194 soda ash, 271 sodium carboxymethyl cellulose, 278 sodium chlorite, 275 sodium citrate, 111 sodium hypochlorite, 275 sodium polyacrylate, 166 sodium polyphosphate, 166 soft lithography, 154 sol-gel technique, 215–16, 270 additives used to prepare hydrophobic or oleophobic sol-gel coatings, 198

334

Index

commercially available silanes, 188 finishing effects, 197–209 improved wear resistance, 206–7 photocatalytic coatings, 207–9 protection against aggressive media, 200–2 ultraviolet protection, colour, photochromic effect, 203–6 water repellence, oil repellence, selfcleaning properties, 197–200 future trends, 209–10 general aspects of textile finishing using (nano-)sols, 190–7 glass fabric that has undergone Martindale abrasion test, 207 glass fibre material coated with hydrophobic hybrid polymer sol, 202 increase in stiffness and mass per unit area of finished fabric with increasing solid content, 192 keywords found in literature when terms ‘sol-gel’ and ‘textile’ are used in search terms, 190 metal oxides prepared by sol-gel technique, 187 organic–inorganic hybrid polymer network basic structural elements that can modify the properties, 189 exhibiting organic and inorganic domains, 188 polyester fabric coated with inorganic nanosol with differing solid content, 193 coated with photochromic coating, 206 modified with photocatalytic coating placed in Petri dishes with dyestuff solution, 208 water uptake after treatment with simple silica sols, 195 principles, 185–90 repellence of fabric treated with standard sol modified with different additives, 200 results of measuring the UPF values of differently treated textiles, 205 simple hybrid polymer sol modified

with different amounts of fluorinated alkoxysilane, 199 steps application, 185–6 curing, 185–6 hydrolysation, 185–6 summary of three steps of sol-gel process, 186 for textile surface modification, 185–210 Trevira CS fabric after being set alight with gas torch, 196 UV-vis spectra of coating based on epoxysilane modified with zinc oxide, 204 wool sample coated with highly hydrophobic sol modified with Aluminium C, 201 Spectra 1000, 252 spin coating technique, 157 spin-coated film gel, 215 sputtering, 61–2 magnetron sputter coating, 61 stand-off mechanism, 291 Staphylococcus aureus, 171, 172 starch, 166 Static Voltmeter R-1020, 119 stearic acid–melamine repellents, 285 stenter frame, 210 steric stabilisation, 166 Sternocera, 140 sulforhodamine, 148 sulfur hexafluoride, 305, 309, 313 superhydrophobic fabrics, 135 super-hydrophobicity, 177 surface analysis, xix surface grafting, 91–105 future trends, 105 properties and applications, 101–4 contact angle measurements, 102 strengths and weaknesses, 104 techniques, 91–7, 99–101 chemical grafting, 92–3 light induced grafting, 100–1 plasma-induced grafting, 96–7, 99–100 radiation-induced grafting, 94–6 surface modification emerging approaches, 318–22

Index enzymes surface modification of textiles, 139–57 textile surface functionalisation by CVD, 126–37 using nanoparticles, 164–82 using sol-gel technique, 185–210 surface modification techniques, 7–9 ionised gas treatments, 8–9 corona discharge, 9 flame treatment, 9 plasma treatment, 8–9 ultraviolet irradiation, 9 wet chemical processing, 7 surface oxidation, 312 surface preparation techniques, 5–7 bleaching, 6–7 desizing, 6 mercerisation, 7 scouring, 6 singeing, 6 surface resistivity, 103 surface-reaction-limited process, 131 synthetic fibres, 4–5 Taguchi method, 177 Tanatex, 146 tartarate, 111 tearing strength, 117–18 Technora, 253 Teflon, 260 Tencel, 142–4 tensile strength, 117 terephthalic acid, 153 tetrachlorosilane, 132, 309 tetracyanoethylene, 257 tetraethoxysilane, 185 textile industries, xix textile sensors, 210 textile surface, xix–xx textile surface modification applications, 280–91 fabric softeners and hand builders, 286–8 mercerisation for cotton fabrics, 280–2 moisture management finishes for performance apparel, 282–4 shrinkage control for wool, 289–91 stain-repellent finishes for cotton and

335

cotton blends, 284–6 surface hydrolysis of polyester, 288–9 by aqueous solutions, 269–93 for composite and filtration applications, 238–64 applications of surface-modified fibres used for filtration, 260–2 future trends, 262–4 surface modification for composites, 239–45 surface modification for filtration, 257–60 surface properties of reinforcing fibres and applications, 246–57 emerging approaches, 318–22 enzymes surface modification, 139–57 future trends, 291–3, 321–2 global apparel fibre market trends from 1980 to 2010, 292 LbL deposition methods for nanomodification, 214–32 conclusions and future trends, 232 LbL deposition on textile surfaces, 221–31 technique, 218–20 mechanisms and chemistries, 270–9 acidic or alkali solutions, 271–3 active substance deposition, 275–7 grafting, 277–8 oxidation or reduction, 273–5 strengths and weaknesses of different techniques, 279 moisture management, 284 nanotechniques, 215–16 new techniques for surface modification, 319–21 atom transfer radical polymerisation, 319–20 biomimetic approaches, 321 ionic liquids, 321 molecular imprinting, 320 ultrasonic waves, 320 orientation of softeners on fibre surface, 287 by plasma treatments, 296–314 applications, 311–13 characterisation, 301–3 future trends, 313–14

336

Index

low-pressure vs atmosphericpressure treatments, 299–300 modifications to textile surfaces, 303–10 nature of plasmas, 297 plasma generation, 297–9 strengths and limitations, 300 surface functionalisation by CVD, 126–37 using nanoparticles, 164–82 commercialisation of nanofinishing in textiles, 180–1 functional properties using nanoparticles, 169–80 future trends, 182 nanoparticles synthesis and characterisation, 165–9 strengths and weaknesses nanotechnology, 182 using sol-gel technique, 185–210 future trends, 209–10 general aspects of textile finishing using (nano-)sols, 190–7 principles, 185–90 sol-gel-based finishing effects, 197–209 textiles surface characterisation methods, 26–54 advanced microscopies, 27–44 advanced spectrometers, 44–50 future trends, 53–4 surface wetting and contact angles, 50–3 surface modification and preparation techniques, 1–22 classification of man-made fibres, 3 future trends, 22 natural fibres, 2–3 recent studies, 9, 11–16, 18–20, 22 surface modification techniques, 7–9 surface preparation techniques, 5–7 synthetic fibres, 4–5 textile fibres classification, 2 thermal CVD, 129, 134 Thermobifida fusca, 152 thermogravimetric analysis, 92 thermolysine, 157 titania, 169, 176, 194, 203, 207

titanium dioxide coating, 68–70 titanium isopropoxide, 169 Toray Industries, 181 transmission electron microscopy, 32–5, 169, 221, 229, 231 with cryo-stage, 35 crystallographic characterisation, 34–5 schematic presentation, 33 surface and interface characterisation, 34 polymide 6 nanofibre, 34 polyvinyl acetate, 34 trichloromethylsilane, 135 Trichoderma reesei, 143 trimethyl aluminium, 136 Tropaeolum sp., 174 Twaron, 253 Twaron aramid fibres, 254 tyrosinase, 149–51 ultra-high-molecular-weight polyethylene, 250 ultrasonic diffusion, 320 ultrasonic extraction, 320 ultrasonic waves, 320 ultraviolet irradiation, 9 ultraviolet protection factor, 173, 203–4 ultraviolet radiation protection, 120–1 untreated, copper-plated & nickelplated polyester fabric, 121 ultraviolet radiation-induced surface grafting, 100–1 universal attenuated total reflectanceFTIR, 46 UPF see ultraviolet protection factor urease, 221 vacuum evaporation, 59–60 vacuum-UV light irradiation, 176 van der Waals, 240, 253 vinyl laurate monomer, 97 viscose, 142–4, 307 wet chemical processing, 7 wettability, 118 wide-angle X-ray diffraction, 225, 229 Wilhelmy technique, 53 dynamic contact angles of PP fibres, 54 wool, 304–5

Index before and after DCCA treatment, 290 shrinkage control, 289–91 additive treatments, 290–1 degradative treatments, 289–90 xerogels, 186 XET see xyloglucan endotransglycosylase XPS see X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy, 11, 46–8, 221, 225, 252, 302

337

PA6-O-MMT composite nanofibre, 47 xyloglucan endotransglycosylase, 147–9 Young’s modulus, 231 zinc dioxide coating, 70–1 AFM image of PET fibre, 72 coating thickness, 71 coating thickness effect on anti-static properties, 73 zinc nitrate hexahydrate, 168 zinc oxide, 168, 203